Fix naming for RHS of named unapply expressions (backport #3163) (#3165)

* Fix naming for RHS of named unapply expressions (#3163)

Previously, the naming plugin would not recurse for the RHS of an
unapply, meaning that vals declared in blocks that ultimately return
some result that is unapplied into some named Data would not be named.

Also, audit and improve the naming mdocs.

(cherry picked from commit 262caa73

)

# Conflicts:
#	docs/src/cookbooks/naming.md

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>