Stop phpDocParam highlight leakage

Problem:  The new Java-like syntax highligting added phpDocComment, et
al.  However phpDocParams highlighted indented html tags in a file with
no doc comments.

Fix: Mark phpDocParams as contained, so phpDocParams must reside in
phpDocComments to receive vim highlights.

Signed-off-by: Justin Charette <charetjc@gmail.com>