Added clarification paragraph to LangRef's documentation of
GlobalVariable about LLVM's assumptions vis-a-vis Global Variable
initial values and Global Variable initializers.

This is in preparation for adding the new keyword
externally_initialized.

Specifically, the patch explains how LLVM optimizes global initializers
by assumign that global variables defined within the module are not
modified from their initial values before the start of the global
initializer.
Added new Global Variable marker ``externally_initialized'' to LangRef.