GN documentation updates.

tools/gn/function_process_file_template.cc

Index: tools/gn/function_process_file_template.cc
diff --git a/tools/gn/function_process_file_template.cc b/tools/gn/function_process_file_template.cc
index 81e3ad34823c695bbd1815f0bd38fc7d7f3276a2..5d4c9d7b4ed5b5dd7bc96ec26ef0976c245a2878 100644
--- a/tools/gn/function_process_file_template.cc
+++ b/tools/gn/function_process_file_template.cc
@@ -20,6 +20,11 @@ const char kProcessFileTemplate_Help[] =
     "  returning the result of applying each template to each source. This is\n"
     "  typically used for computing output file names from input files.\n"
     "\n"
+    "  In most cases, get_target_outputs() will give the same result with\n"
+    "  shorter, more maintainable code. This function should only be used\n"
+    "  when that function can't be used (like there's no target or the target\n"
+    "  is defined in another build file).\n"
+    "\n"
     "Arguments:\n"
     "\n"
     "  The source_list is a list of file names.\n"