contrib: rename revsetbenchmarks.txt to 'base-revsets.txt'

We rename the file and document its purpose. We'll be introducing another file
gathering revsets useful for benchmark of the predicate themsleves in a coming
changesets.

contrib/base-revsets.txt

@@ -1,3 +1,20 b''
+# Base Revsets to be used with revsetbenchmarks.py script
+#
+# The goal of this file is to gather a limited amount of revsets that allow a
+# good coverage of the internal revsets mechanisms.  Revsets included should not
+# be selected for their individual implementation, but for what they reveal of
+# the internal implementation of smartsets classes (and their interactions).
+#
+# Use and update this file when you change internal implementation of these
+# smartsets classes. Please include a comment explaining what each of your
+# addition is testing. Also check if your changes to the smartset class makes
+# some of the tests inadequate and replace them with a new one testing the same
+# behavior.
+#
+# The current content of this file is currently likely not reaching this goal
+# entirely, feel free, to audit its content and comment on each revset to
+# highlight what internal mechanisms they test.
+
 all()
 draft()
 ::tip