docs: fixup envvar-directive syntax

This directive needs a newline following it to render correctly.

While we're at it, fixup the incorrect indent for one of the
descriptions.

Fixes: 0c58ad3e32 ("docs: use envvar directive more")
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20640>

docs/envvars.rst

@@ -550,6 +550,7 @@ Intel driver environment variables
    assembly.
 
 .. envvar:: INTEL_SIMD_DEBUG
+
    a comma-separated list of named flags, which control simd dispatch widths:
 
    ``fs8``
@@ -1492,6 +1493,7 @@ PowerVR driver environment variables
 ------------------------------------------------
 
 .. envvar:: PVR_DEBUG
+
    A comma-separated list of debug options. Use `PVR_DEBUG=help` to
    print a list of available options.