fileset: don't suggest private or undocumented queries

Unlike revsets, it looks like all of the filesets are documented, so there's
really nothing to test.  This is aimed more at parity with revsets and
future-proofing.

mercurial/fileset.py

@@ -189,7 +189,11 b' def clean(mctx, x):'
 def func(mctx, a, b):
     if a[0] == 'symbol' and a[1] in symbols:
         return symbols[a[1]](mctx, b)
-    raise error.UnknownIdentifier(a[1], symbols.keys())
+
+    keep = lambda fn: getattr(fn, '__doc__', None) is not None
+
+    syms = [s for (s, fn) in symbols.items() if keep(fn)]
+    raise error.UnknownIdentifier(a[1], syms)
 
 def getlist(x):
     if not x: