Docs: Correcting section on how to escape quotes in strings.

The behavior for single and double-quotes is different--I expanded
this section to note how they differ.

(New version of a Gerrit CL I abandoned because of a commit mess-up.)

See first comment for doc stage location.

bug: 19959941
Change-Id: I417546a473f0ebe76a6e4102b87883a85365ac26