Apply TCO to recurse/1, add recurse/2; tweak docs

Signed-off-by: Nicolas Williams <nico@cryptonector.com>

builtin.c

@@ -928,9 +928,13 @@ static const char* const jq_builtins[] = {
   "def del(f): delpaths([path(f)]);",
   "def _assign(paths; value): value as $v | reduce path(paths) as $p (.; setpath($p; $v));",
   "def _modify(paths; update): reduce path(paths) as $p (.; setpath($p; getpath($p) | update));",
-  "def recurse(f): ., (f | select(. != null) | recurse(f));",
+
+  // recurse
+  "def recurse(f): def r: ., (f | select(. != null) | r); r;",
+  "def recurse(f; cond): def r: ., (f | select(cond) | r); r;",
   "def recurse: recurse(.[]?);",
   "def recurse_down: recurse;",
+
   "def to_entries: [keys[] as $k | {key: $k, value: .[$k]}];",
   "def from_entries: map({(.key): .value}) | add | .//={};",
   "def with_entries(f): to_entries | map(f) | from_entries;",