Remove mapscript docs referencing 8.4 functionality from 8.2 branch

[geographika]

Documentation builds for the 8.2 branch are reporting the following errors:

AttributeError: module 'mapscript' has no attribute 'MS_COMPOP_HSL_HUE'
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.12/x64/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 210, in import_object
    obj = attrgetter(obj, mangled_name)
...

I think this is due to #914 being merged in May 2024, prior to the 8.2 documentation branch being created. When the branch was created from main this pull request was included.

The references to 8.4 functionality are live at https://www.mapserver.org/mapfile/composite.html (which isn't an issue). This pull requests fixes the 8.2 branch to avoid the above Python errors.