RE: tags for functions

[Drew Adams]

1. When the thing documented in *Help* is also in the manual, then we have a
link to the manual from the *Help* buffer.

2. The manual should have such a `See also' cross reference, if any place
should.

3. When both the thing doc'd in *Help* and the thing xref'd are in the manual,
#1 and #2 would take care of such cross-referencing. Admittedly, passing through
the manual represents an indirection, but typically a fruitful one.

Dunno if it's worth adding this for cases that are not covered by the manual. It
sounds like a helpful feature. I can't speak to the implementation cost. (There
is also a doc-maintenance cost to consider.)