doc common: Draft documentation for C++ header APIs

[JSUYA]

• Description :
  Write documentation for some of the APIs that have already been cleared up.

• Issue Tickets (if any) :
  API Documentation  #27

[github-actions]

CODING STYLE CHECK:

thorvg.h:133:  public: should be indented +1 space inside class Paint  [whitespace/indent] [3]
thorvg.h:217:  public: should be indented +1 space inside class Fill  [whitespace/indent] [3]
thorvg.h:278:  public: should be indented +1 space inside class Canvas  [whitespace/indent] [3]
thorvg.h:322:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:354:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:388:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:645:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:703:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:740:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:772:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:800:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:801:  Do not leave a blank line after "public:"  [whitespace/blank_line] [3]
Done processing thorvg.h
Total errors found: 12

[github-actions]

CODING STYLE CHECK:

thorvg.h:133:  public: should be indented +1 space inside class Paint  [whitespace/indent] [3]
thorvg.h:217:  public: should be indented +1 space inside class Fill  [whitespace/indent] [3]
thorvg.h:278:  public: should be indented +1 space inside class Canvas  [whitespace/indent] [3]
thorvg.h:322:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:354:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:388:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:645:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:703:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:740:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:772:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:800:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:801:  Do not leave a blank line after "public:"  [whitespace/blank_line] [3]
Done processing thorvg.h
Total errors found: 12

[github-actions]

CODING STYLE CHECK:

thorvg.h:133:  public: should be indented +1 space inside class Paint  [whitespace/indent] [3]
thorvg.h:217:  public: should be indented +1 space inside class Fill  [whitespace/indent] [3]
thorvg.h:278:  public: should be indented +1 space inside class Canvas  [whitespace/indent] [3]
thorvg.h:322:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:354:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:388:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:645:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:703:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:740:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:772:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:800:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
Done processing thorvg.h
Total errors found: 11

[github-actions]

CODING STYLE CHECK:

thorvg.h:133:  public: should be indented +1 space inside class Paint  [whitespace/indent] [3]
thorvg.h:217:  public: should be indented +1 space inside class Fill  [whitespace/indent] [3]
thorvg.h:278:  public: should be indented +1 space inside class Canvas  [whitespace/indent] [3]
thorvg.h:322:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:354:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:388:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:645:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:703:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:740:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:772:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
thorvg.h:800:  public: should be indented +1 space inside class final  [whitespace/indent] [3]
Done processing thorvg.h
Total errors found: 11

[hermet]

@mgrudzinska please have a look this patch & contribute additional patches for the unsatisfied stuff.

[mgrudzinska]

@hermet sure! I'll do it