add docs for 'use nqp;'

Closes #796

doc/Language/pragmas.pod6

@@ -50,7 +50,24 @@ not yet rendered properly.
 
 =item X<B<newline>|newline> [TBD]
 
-=item X<B<nqp>|nqp> [TBD]
+=item X<B<nqp>|nqp>
+
+Use at your own risk.
+
+This is a Rakudo-specific pragma. With it, Rakudo provides access to the
+L<nqp opcodes|https://github.com/perl6/nqp/blob/master/docs/ops.markdown>
+in a top level namespace, for example:
+
+    use nqp;
+    nqp::say("hello world");
+
+Uses the underlying nqp C<say> opcode instead of the Perl 6 routine. This
+pragma may make your code rely on a particular version of nqp, and since
+that code is not part of the Perl 6 specification, it's not guaranteed to
+be stable. You may find a large number of usages in the Rakudo core,
+which are used to make the core functionality as fast as possible.
+Future optimizations in the code generation of Rakudo may obsolete these
+usages.
 
 =item X<B<parameters>|parameters> [NYI]
 

xt/words.pws

@@ -660,6 +660,7 @@ oo
 oop
 opaquepointer
 opcode
+opcodes
 opendir
 openforwriting
 opstring