[texinfo] Change formatting of @strong and @emph in Info output.

Using strong or emphasis to markup parameter names is quite common and the
default formatting for *strong* and _emph_ often creates confusing output.

They are now formatted like `literals'.

sphinx/writers/texinfo.py

@@ -46,6 +46,8 @@ TEMPLATE = """\
 @exampleindent %(exampleindent)s
 @finalout
 %(direntry)s
+@definfoenclose strong,`,'
+@definfoenclose emph,`,'
 @c %%**end of header
 
 @copying