doc: make links for hg role
The system has a bit of smartness:
:hg:`update` -> links to hg.1.html#update
:hg:`help diffs` -> links to hg.1.html#diffs
./wrong.py:1:
> def toto( arg1, arg2):
gratuitous whitespace in () or []
./wrong.py:2:
> del(arg2)
del isn't a function
./wrong.py:3:
> return ( 5+6, 9)
missing whitespace in expression
gratuitous whitespace in () or []
./quote.py:5:
> '"""', 42+1, """and
missing whitespace in expression