docs: update MESA_DEBUG envvar documentation.

silent, flush, incomplete_tex and incomplete_fbo flags were not
documented (see src/mesa/main.debug.c for more info).

FP is not checked anymore.

v2 (Brian Paul):
 * MESA_DEBUG accepts a comma-separated list of parameters.
 * Clarify how MESA_DEBUG behaves with mesa debug and release builds.
 * Updated wording.

v3: Better wording for one paragraph (Brian Paul)

Reviewed-by: Brian Paul <brianp@vmware.com>

docs/envvars.html

@@ -50,8 +50,17 @@ sometimes be useful for debugging end-user issues.
    if the application generates a GL_INVALID_ENUM error, a corresponding error
    message indicating where the error occurred, and possibly why, will be
    printed to stderr.<br>
-   If the value of MESA_DEBUG is 'FP' floating point arithmetic errors will
-   generate exceptions.
+
+   For release builds, MESA_DEBUG defaults to off (no debug output).
+
+   MESA_DEBUG accepts the following comma-separated list of named
+   flags, which adds extra behaviour to just set MESA_DEBUG=1:
+   <ul>
+     <li>silent - turn off debug messages. Only useful for debug builds.</li>
+     <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>
+   </ul>
 <li>MESA_LOG_FILE - specifies a file name for logging all errors, warnings,
 etc., rather than stderr
 <li>MESA_TEX_PROG - if set, implement conventional texture env modes with