Fix joint limits namespace wording in docstrings (backport #3445)

[mergify]

Summary

• Update the return-value documentation for two get_joint_limits overloads.
• Replace the slash-based parameter wording with the dotted
  joint_limits.joint_name namespace format.
• Clarify that the general joint-limits overload succeeds when at least one
  recognized parameter exists under the namespace.

Fixes #3419

Testing

• git diff --check
• py -m pre_commit run
  Documentation-only change; no runtime behavior was modified.

  ---

  This is an automatic backport of pull request Fix joint limits namespace wording in docstrings #3445 done by Mergify.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.42%. Comparing base (0194fdd) to head (47f175e).

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #3447      +/-   ##
==========================================
- Coverage   89.44%   89.42%   -0.03%     
==========================================
  Files         161      161              
  Lines       20567    20568       +1     
  Branches     1630     1630              
==========================================
- Hits        18397    18392       -5     
- Misses       1500     1506       +6     
  Partials      670      670              

Flag	Coverage Δ
unittests	89.42% <ø> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
...its/include/joint_limits/joint_limits_rosparam.hpp	59.42% <ø> (ø)

... and 2 files with indirect coverage changes

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