runtime: document scope footgun in watch_object

[clux]

it is possible to use Api::all with a namespaced object with watch_object which can lead to duplicate matches.

This is not desirable and we can't avoid this directly (for at least as long as we use Api and not client_ext in this fn), so have added a warning to the function.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.1%. Comparing base (6ce3978) to head (61b324d).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1510     +/-   ##
=======================================
+ Coverage   75.1%   75.1%   +0.1%     
=======================================
  Files         78      78             
  Lines       6864    6865      +1     
=======================================
+ Hits        5150    5151      +1     
  Misses      1714    1714             

Files	Coverage Δ
kube-runtime/src/watcher.rs	42.0% <100.0%> (+0.3%)	⬆️