Fix joint limits namespace wording in docstrings

[mhfakouri]

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (2296fd8) to head (6c69f70).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3445      +/-   ##
==========================================
- Coverage   89.56%   89.51%   -0.05%     
==========================================
  Files         164      164              
  Lines       21270    21270              
  Branches     1656     1656              
==========================================
- Hits        19050    19040      -10     
- Misses       1520     1529       +9     
- Partials      700      701       +1     

Flag	Coverage Δ
unittests	89.51% <ø> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[christophfroehlich]

Thank you!