docs(behavior_path_planner): add explanation for dynamic lateral acceleration

[purewater0901]

Description

Update the lane change document by adding an explanation about the dynamic lateral acceleration decider.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.05 🎉

Comparison is base (f09e575) 14.45% compared to head (b9688dc) 14.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3965      +/-   ##
==========================================
+ Coverage   14.45%   14.50%   +0.05%     
==========================================
  Files        1456     1456              
  Lines      102538   103259     +721     
  Branches    29652    30203     +551     
==========================================
+ Hits        14819    14982     +163     
- Misses      71568    71947     +379     
- Partials    16151    16330     +179     

Flag	Coverage Δ		*Carryforward flag
differential	12.88% <ø> (?)
total	14.45% <ø> (ø)		Carriedforward from f09e575

*This pull request uses carry forward flags. Click here to find out more.

see 6 files with indirect coverage changes

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