Improve doc about "maint print c-tdesc"

Improve doc about "maint print c-tdesc"

In my patch extending command "maint print c-tdesc"
(https://sourceware.org/ml/gdb-patches/2017-06/msg00286.html), Eli
raised some questions on the existing doc to this command.  It is not
very clear, so this patch improves it.  Eli approved it.

gdb/doc:

2017-07-18  Yao Qi  <yao.qi@linaro.org>

	* gdb.texinfo (Maintenance Commands): Improve the doc to
	command "maint print c-tdesc".