escape HTML elements in docblock with double quotes

escape HTML elements in docblock with double quotes

Doxygen choke on text enclosed by '<' and '>' since it tries to
interpret them as HTML or XML elements. This patch adds double quotes
in includes/api/*.php files around the two following strings:

 <Firstname>.<Lastname>@gmail.com
 <Firstname><Lastname>@gmail.com

Which becomes:

 "<Firstname>.<Lastname>@gmail.com"
 "<Firstname><Lastname>@gmail.com"

Tested locally, it prevents doxygen 1.8.0 related warnings.

Change-Id: I36d82eb3fd4989ee3ffc65b0b527b83711d1ba69