[doc] Describe the header guard style

[mtrofin]

Summary

clang-tidy's llvm-header-guard rule references the LLVM style - where it's
missing.

llvm/docs/CodingStandards.rst

 
 The main body is a `Doxygen <http://www.doxygen.nl/>`_ comment (identified by
 the ``///`` comment marker instead of the usual ``//``) describing the purpose
 of the file.  The first sentence (or a passage beginning with ``\brief``) is
 used as an abstract.  Any additional information should be separated by a blank
 line.  If an algorithm is based on a paper or is described in another source,
 provide a reference.
 
+Header Guard
+""""""""""""
+
+The header file's guard should be the all-caps path that a user of this header
+would #include, using '_' instead of path separator and extension marker. 
+For example, the header file
+``llvm/include/llvm/Analysis/Utils/Local.h`` would be ``#include``-ed as 
+``#include "llvm/Analysis/Utils/Local.h"``, so its guard is 
+``LLVM_ANALYSIS_UTILS_LOCAL_H``.
+
 Class overviews
 """""""""""""""
 
 Classes are a fundamental part of an object-oriented design.  As such, a
 class definition should have a comment block that explains what the class is
 used for and how it works.  Every non-trivial class is expected to have a
 ``doxygen`` comment block.