New @qcode macro for documentation

[Rik]

8/19/13

All,

I added a new macro, @qcode, to the documentation for the more specialized
situation of quoted code samples.  This actually happens fairly often
whenever you have string options such as "on"/"off" or refer to handle
graphics properties such as "position".

The Texinfo sample @code{xxx} expands to `xxx' in the Info/plaintext
representation.  This looks rather ugly with quoted text: `"on"'/`"off"'. 
Instead the new macro passes the quoted text directly through to
Info/plaintext, but preserves the nice medium weight font which is used in TeX.

The mnemonic to remember this new macro is q(uoted)code.

--Rik

Changeset: http://hg.savannah.gnu.org/hgweb/octave/rev/bc924baa2c4e