Clarify behavior of "default" submit types

At a project level, the configured submit type is the default in the
sense that it is used when a submit type rule does not override the
project's default.

At the gerrit.config level, the defaultSubmitType is only used during
project creation time.

Change-Id: I730e37bc6bbb9bdfd2b70350e303e0878bf105c1

Documentation/config-gerrit.txt

@@ -3753,7 +3753,10 @@ are `MERGE_IF_NECESSARY`, `FAST_FORWARD_ONLY`, `REBASE_IF_NECESSARY`,
 +
 For more details see link:project-configuration.html#submit_type[Submit Types].
 +
-By default, `MERGE_IF_NECESSARY`.
+This submit type is only applied at project creation time if a submit type is
+omitted from the link:rest-api-projects.html#project-input[ProjectInput]. If the
+submit type is unset in the project config at runtime, it defaults to
+link:project-configuration.html#merge_if_necessary[`MERGE_IF_NECESSARY`].
 
 [[repository.name.ownerGroup]]repository.<name>.ownerGroup::
 +

Documentation/project-configuration.txt

@@ -70,7 +70,8 @@ tip of the destination branch at submit time.
 [[merge_if_necessary]]
 * Merge If Necessary
 +
-This is the default for a new project.
+This is the default for new projects, unless overridden by a global
+link:config-gerrit.html#repository.name.defaultSubmitType[`defaultSubmitType` option].
 +
 If the change being submitted is a strict superset of the destination
 branch, then the branch is fast-forwarded to the change.  If not,