help: explain what the term "revset" means

We refer to revsets in a few places (e.g. in "hg help config"), but we
never explained what they are. Until now.

mercurial/help/revisions.txt

@@ -33,7 +33,7 b' Specifying multiple revisions'
 =============================
 
 Mercurial supports a functional language for selecting a set of
-revisions.
+revisions. Expressions in this language are called revsets.
 
 The language supports a number of predicates which are joined by infix
 operators. Parenthesis can be used for grouping.