docs: removed the '--with-sha1' requirement from shading.html

docs: removed the '--with-sha1' requirement from shading.html

The configuration option --with-sha1 is no longer required for the
MESA_SHADER_READ_PATH, MESA_SHADER_DUMP_PATH environment variables
to take effect.

1- removed the "--with-sha1" sentence from docs/shading.html
2- added an extra note: that the corresponding dumped and replacement
shaders must have the same filenames for the feature to take effect.

Acked-by: Tapani Pälli <tapani.palli@intel.com>