Since commit c2dc7e0f53 (Help: Convert genex documentation to sphinx
domain objects, 2021-01-15) we can use `:genex:` cross-references to
link to named generator expressions. Update some places to do this.
This is meant to demonstrate the capability, not as a comprehensive
sweep.
When the path to a header file is specified using a generator
expression, evaluation of the genex must produce an absolute path.
Update our documented example and add a test covering the case.
Fixes: #20617
When passing a `<header>` inside a generator expression, the closing
`>` needs to be encoded as `$<ANGLE-R>`. Add an example to the docs.
Fixes: #19940
