docs: document MESA_DEBUG=context

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

docs/envvars.html

@@ -60,6 +60,8 @@ sometimes be useful for debugging end-user issues.
      <li>flush - flush after each drawing command</li>
      <li>incomplete_tex - extra debug messages when a texture is incomplete</li>
      <li>incomplete_fbo - extra debug messages when a fbo is incomplete</li>
+     <li>context - create a debug context (see GLX_CONTEXT_DEBUG_BIT_ARB) and
+         print error and performance messages to stderr (or MESA_LOG_FILE).</li>
    </ul>
 <li>MESA_LOG_FILE - specifies a file name for logging all errors, warnings,
 etc., rather than stderr