Merge topic 'clarify-add_custom_command-TARGET-scope'

d257d681 add_custom_command: Clarify error when TARGET is out of scope (#15681)
4d53e0a7 Help: Clarify `add_custom_command(TARGET)` scope (#15681)
8c615af4 Help: Clarify policy `CMP0040` documentation (#15681)
63c5808f Help: Clarify scope of `if(TARGET)` expression
a336e438 Help: Improve markup in `if` command documentation
88968265 Help: Improve markup in `get_target_property` documentation