[docs] Mention character array constants in docs/LangRef.rst

Summary:
They were used in the 'Module Structure' example but weren't otherwise
documented.

Credit to Reed Kotler for noticing.

Reviewers: hans

Reviewed By: hans

Subscribers: hans, llvm-commits

Differential Revision: http://reviews.llvm.org/D5191

llvm-svn: 217583