cmake: say 'absolute path' in option descriptions and docs

[vszakats]

To not have to guess. Also to sync with autotools, which already uses
this wording.

Also:

• replace the stray term 'folder' with 'directory' for consistency.
• store help text in a temp variable to avoid overly long strings
  (mandatory in CMake <4.2.0 and can't be trivially split), also
  to avoid repeating this string 4 times.

Ref: https://cmake.org/cmake/help/v4.2/command/set.html

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This will be a tedious rebase in #16973.

The ([A-Za-z0-9-]+) include directory.
Absolute path to \1 include directory.