Help: Remove stray content from 'add_custom_command' docs

Remove leftover generator expression documentation that is now in
the cmake-generator-expressions(7) manual.
Help: Revise and format 'add_custom_command' docs

Format the reStructuredText markup manually.  Organize the command
options into a definition list.  Use inline markup to cross-reference
related documents.
Help: Document add_custom_command PRE_BUILD/PRE_LINK for exe/lib only (#15059)

State explicitly that the PRE_LINK mode is not for targets created by
the add_custom_target command.  The existing wording for PRE_BUILD being
treated as PRE_LINK by non-VS generators will now imply this restriction
for PRE_BUILD too.
Merge topic 'doc-add_custom_command' into next

5bf1c5cc Help: Document add_custom_command PRE_BUILD/PRE_LINK for exe/lib only (#15059)
2a58c872 Help: Revise and format 'add_custom_command' docs
cb88742d Help: Remove stray content from 'add_custom_command' docs