Fix documentation and method name for FootnoteProperties

[mdupont]

Description

• Fixed documentation about FootnoteProperties which contained an invalid code snippet, so that people can actually use the code:
  • it now uses the correct class names
  • no more typo in variable name usage
  • now uses a Footnote position parameter that works when outputting Word (POSITION_BENEATH_TEXT). POSITION_DOC_END only works for Endnotes.
• Fixed bogus method name $section->addFootnotePropoperties():
  • added a method with the correct spelling $section->addFootnoteProperties()
  • former method still works but is marked as deprecated
  • migrated code to use the correct method name

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[coveralls]

Coverage remained the same at 94.633% when pulling cb3e211 on mdupont:footnotes-typo into dfea4e1 on PHPOffice:develop.