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 topic 'doc-add_compile_options' into next

352cbb6e Help: Reference add_compile_options from add_definitions (#15225)