Mention EBUSY error in troubleshooting guide

[borine]

Documentation update

Make it clear that BlueALSA does not permit multiple simultaneous connections to the same PCM.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 64.19%. Comparing base (c18e812) to head (422a2c3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #701      +/-   ##
==========================================
- Coverage   64.21%   64.19%   -0.02%     
==========================================
  Files          93       93              
  Lines       14809    14809              
  Branches     2385     2385              
==========================================
- Hits         9509     9507       -2     
- Misses       5194     5196       +2     
  Partials      106      106              

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