FindCurses: Include CheckLibraryExists before using it (#15220)

Include the module at the top unconditionally so that all code paths can
use it.
Help: Clarify add_compile_options documentation (#15225)

Explain that it affects only targets added after the call.  Fix the link
to the COMPILE_OPTIONS directory propert documentation.  Update the
latter to explain that it is used to initialize the corresponding target
property when a target is created.
Help: Reference add_compile_options from add_definitions (#15225)

When we explain in add_definitions documentation that it is intended
only for preprocessor definitions, link to add_compile_options for
adding other flags.  Also explicitly mention that the order of
add_definitions calls with respect to target creation does not matter.
This differs from the behavior now explicitly stated in the
documentation of add_compile_options.
Merge branch 'FindCurses-include-CheckLibraryExists' into release
Merge branch 'doc-add_compile_options' into release
Documentation: Fix minor typo thecommand -> the command
Merge branch 'doc-add_library-typo' into release