Add scm_revision to project updates and cleanup

Add validation around prompted scm_branch requiring
  project allow_override field to be true

Updated related process isolation docs

Fix invalid comarision in serializer

from PR review, clarify pre-check logging, minor docs additions

docs/process_isolation.md

@@ -11,7 +11,7 @@ Tower 3.5 forward uses the process isolation feature in ansible runner to achiev
 By default `bubblewrap` is enabled, this can be turned off via Tower Config or from a tower settings file:
 
     AWX_PROOT_ENABLED = False
-    
+
 Process isolation, when enabled, will be used for the following Job Types:
 
 * Job Templates - Launching jobs from regular job templates
@@ -30,11 +30,18 @@ If there is other information on the system that is sensitive and should be hidd
 or by updating the following entry in a tower settings file:
 
     AWX_PROOT_HIDE_PATHS = ['/list/of/', '/paths']
-    
+
 If there are any directories that should specifically be exposed that can be set in a similar way:
 
     AWX_PROOT_SHOW_PATHS = ['/list/of/', '/paths']
-    
+
 By default the system will use the system's tmp dir (/tmp by default) as it's staging area. This can be changed:
 
     AWX_PROOT_BASE_PATH = "/opt/tmp"
+
+### Project Folder Isolation
+
+Starting in AWX versions above 6.0.0, the project folder will be copied for each job run.
+This allows playbooks to make local changes to the source tree for convenience,
+such as creating temporary files, without the possibility of interference with
+other jobs.