[man page] Document -gline-tables-only in the clang man page.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275076 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/clang.rst

@@ -253,22 +253,32 @@ Code Generation Options
 
       Currently equivalent to :option:`-O3`
 
-.. option:: -g
+.. option:: -g, -gline-tables-only, -gmodules
 
-  Generate debug information.  Note that Clang debug information works best at -O0.
+  Control debug information output.  Note that Clang debug information works
+  best at :option:`-O0`.  When more than one option starting with `-g` is
+  specified, the last one wins:
 
-.. option:: -gmodules
+    :option:`-g` Generate debug information.
 
-  Generate debug information that contains external references to
-  types defined in clang modules or precompiled headers instead of
-  emitting redundant debug type information into every object file.
-  This option implies :option:`-fmodule-format=obj`.
+    :option:`-gline-tables-only` Generate only line table debug information. This
+    allows for symbolicated backtraces with inlining information, but does not
+    include any information about variables, their locations or types.
+
+    :option:`-gmodules` Generate debug information that contains external
+    references to types defined in Clang modules or precompiled headers instead
+    of emitting redundant debug type information into every object file.  This
+    option transparently switches the Clang module format to object file
+    containers that hold the Clang module together with the debug information.
+    When compiling a program that uses Clang modules or precompiled headers,
+    this option produces complete debug information with faster compile
+    times and much smaller object files.
+
+    This option should not be used when building static libraries for
+    distribution to other machines because the debug info will contain
+    references to the module cache on the machine the object files in the
+    library were built on.
 
-  This option should not be used when building static libraries for
-  distribution to other machines because the debug info will contain
-  references to the module cache on the machine the object files in
-  the library were built on.
-  
 .. option:: -fstandalone-debug -fno-standalone-debug
 
   Clang supports a number of optimizations to reduce the size of debug