Update help docs for the global command

I noticed that COMPOSER_HOME/vendor/bin was wrong. Correcting.
Add a note about customizations
Improve global command to support short command resolution
Validate constraints in require command, fixes #2197
Better wording in a note
Merge pull request #2209 from mgrabovsky/patch-1

Better wording in a note
Merge pull request #2205 from unn/patch-1

Update help docs for the global command