docs/devinfo: Expound on helpful extension tips

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/devinfo.html

@@ -684,6 +684,8 @@ To add a new GL extension to Mesa you have to do at least the following.
 </li>
 <li>
    Add a new entry to the <code>gl_extensions</code> struct in mtypes.h
+   if the extension requires driver capabilities not already exposed by
+   another extension.
 </li>
 <li>
    Add a new entry to the src/mesa/main/extensions_table.h file.
@@ -697,6 +699,11 @@ To add a new GL extension to Mesa you have to do at least the following.
    If the new extension adds new GL state, the functions in get.c, enable.c
    and attrib.c will most likely require new code.
 </li>
+<li>
+   To determine if the new extension is active in the current context,
+   use the auto-generated _mesa_has_##name_str() function defined in
+   src/mesa/main/extensions.h.
+</li>
 <li>
    The dispatch tests check_table.cpp and dispatch_sanity.cpp
    should be updated with details about the new extensions functions. These