util: make capacity a public attribute on lrucachedict
So others can query it. Useful for operations that may want to verify
the cache has capacity for N items before it performs an operation that
may cause cache eviction.
Differential Revision: https://phab.mercurial-scm.org/D4499
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors