NO BUG - Document buildapp in mozinfo

DONTBUILD (NPOTB)

build/docs/mozinfo.rst

@@ -59,6 +59,12 @@ bits
 
    Optional.
 
+buildapp
+   The path to the XUL application being built.
+
+   For desktop Firefox, this is ``browser``. For Fennec, it's
+   ``mobile/android``. For B2G, it's ``b2g``.
+
 crashreporter
    Whether the crash reporter is enabled for this build.