client: improve documentation of target strings

[dfawley]

RELEASE NOTES:

• client: improve documentation of NewClient and CanonicalTarget by providing examples.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.29%. Comparing base (0003b4f) to head (4f5c3aa).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8078      +/-   ##
==========================================
- Coverage   82.32%   82.29%   -0.03%     
==========================================
  Files         387      387              
  Lines       39064    39064              
==========================================
- Hits        32159    32148      -11     
- Misses       5593     5600       +7     
- Partials     1312     1316       +4     

Files with missing lines	Coverage Δ
clientconn.go	90.29% <ø> (ø)

... and 22 files with indirect coverage changes