doc: Document why unused parameters in Raptor API should NOT be deleted.

[t2gran]

Summary

Small code doc change to Raptor.

Issue

Related to #4410

Unit tests

Not relevant

Documentation

✅

[codecov-commenter]

Codecov Report

Merging #4413 (ee919be) into dev-2.x (f791309) will increase coverage by 0.05%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             dev-2.x    #4413      +/-   ##
=============================================
+ Coverage      58.04%   58.09%   +0.05%     
- Complexity     11045    11067      +22     
=============================================
  Files           1454     1455       +1     
  Lines          58725    58748      +23     
  Branches        6760     6765       +5     
=============================================
+ Hits           34085    34129      +44     
+ Misses         22605    22581      -24     
- Partials        2035     2038       +3     

Impacted Files	Coverage Δ
...oradapter/transit/request/TripPatternForDates.java	75.71% <0.00%> (-1.90%)	⬇️
.../algorithm/raptoradapter/router/TransitRouter.java	75.89% <0.00%> (-0.22%)	⬇️
...tripplanner/ext/traveltime/TravelTimeResource.java	0.00% <0.00%> (ø)
.../transit/raptor/api/transit/RaptorTripPattern.java	100.00% <0.00%> (ø)
...rithm/raptoradapter/transit/cost/McCostParams.java	61.90% <0.00%> (ø)
...aptoradapter/transit/cost/McCostParamsBuilder.java	100.00% <0.00%> (ø)
...dapter/transit/frequency/FrequencyAlightEvent.java	0.00% <0.00%> (ø)
...pter/transit/frequency/FrequencyBoardingEvent.java	0.00% <0.00%> (ø)
...er/transit/frequency/TripFrequencyBoardSearch.java	0.00% <0.00%> (ø)
...r/transit/frequency/TripFrequencyAlightSearch.java	0.00% <0.00%> (ø)
... and 15 more

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