doc: Fix incorrect option "disassociate" in "checkoutSCM" git section

[kenlau46]

Bob return failure when use disassociate in checkoutSCM section

Parse error: Error while validating recipes/bob.yaml: Key 'checkoutSCM' error:
Wrong key 'disassociate' in {'scm': 'git', 'url': '[email protected]:BobBuildTool/bob.git', 'branch': 'master', 'singleBranch': True, 'shallow': 1, 'disassociate': True}

Browse through the source code and dissociate is defined and used instead.

Also fix typo and add man page section that explains the option

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.84%. Comparing base (c1146f8) to head (f11e2fb).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
- Coverage   88.86%   88.84%   -0.03%     
==========================================
  Files          48       48              
  Lines       15468    15467       -1     
==========================================
- Hits        13745    13741       -4     
- Misses       1723     1726       +3     

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[jkloetzke]

Good catch. The documentation obviously misspelled the option. Thanks!