Add clarification to GridQubit neighbors function

[dstrain115]

• An internal bug noted that there could be confusion if you assumed that GridQubit.neighbors() returned only the neighbors on a specific device layout.
• This changes only the documentation to point out that this function is device-agnostic.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (c434f94) to head (d1aaf58).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7521      +/-   ##
==========================================
- Coverage   98.68%   98.68%   -0.01%     
==========================================
  Files        1091     1091              
  Lines       96948    96948              
==========================================
- Hits        95674    95673       -1     
- Misses       1274     1275       +1     

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