Improve documentation of submit.mergeContent setting in project.config
Improve documentation of submit.mergeContent setting in project.config

* explain what a path conflict is and include an example
* explain the extra work and user confusion that comes with disabling
  content merges
* explain what's gained by disabling content merges
* discourage disabling content merge
* note that this setting is not relevant when fast-forward-only is used
  as the submit action

Release-Notes: skip
Bug: Google b/240397771
Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: If3e2b29efea166be13069e35ed22f343f478957d

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