add_definitions: Don't document genex support.

This command does not support generator expressions. The documentation
was mistakenly extended to claim it in commit v3.0.0-rc1~60^2~3 (Help: Mark
up the buildsystem commands documentation, 2014-02-03).
CMP0043: Document old and new interfaces for setting directory property.
Merge topic 'CMP0043-dir-prop'

38df26bc CMP0043: Document old and new interfaces for setting directory property.
Merge topic 'add_definitions-no-genex'

1a9316a8 add_definitions: Don't document genex support.