docs: hip-991 protobuf documentation fix

[kimbor]

Update the protobuf docs for consensus_update_topic.proto to clarify the following:
-Only the admin key has to sign if the custom fee key is updated.

Related issue(s):
Fixes #15566

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (hip-991-topic-fees@3b741ac). Learn more about missing BASE report.

Additional details and impacted files

@@                  Coverage Diff                  @@
##             hip-991-topic-fees   #15567   +/-   ##
=====================================================
  Coverage                      ?   58.06%           
  Complexity                    ?    21532           
=====================================================
  Files                         ?     2775           
  Lines                         ?   109025           
  Branches                      ?    11174           
=====================================================
  Hits                          ?    63308           
  Misses                        ?    41857           
  Partials                      ?     3860           

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 3b741ac1	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (3b741ac)	Report Missing	Report Missing	Report Missing
Head commit (2a4cc86)	108842	67058	61.61%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#15567)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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[netopyr]

LGTM - thanks @kimbor