Clarify that GEN_KW does not support FORWARD_INIT

[mortalisk]

Issue
Resolves #5156

Approach
Updates the documentation

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.
• Updated documentation
• Ensured new behaviour is tested

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #5232 (8294934) into main (4331415) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5232   +/-   ##
=======================================
  Coverage   72.33%   72.34%           
=======================================
  Files         396      396           
  Lines       27447    27447           
  Branches     2007     2007           
=======================================
+ Hits        19855    19857    +2     
+ Misses       7028     7026    -2     
  Partials      564      564           

see 1 file with indirect coverage changes

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[andreas-el]

It's in line with what the code does! 👍