Improve required.method.not.called diagnostic messaging

[thisisalexandercook]

fixes #1542.

This PR improves Resource Leak Checker diagnostics for required.method.not.called to make them clearer and less misleading. It also normalizes reason text formatting (lowercase fragments and consistent spacing).

Sample output from latest commit:

checker/tests/resourceleak/RequiredMethodNotCalledMessage.java:19: error: [required.method.not.called] Required method close may not have been invoked on resource or any of its aliases.
        DemoResource resource = new DemoResource();
                     ^
  The type of object is: RequiredMethodNotCalledMessage.DemoResource.
  Reason for going out of scope: regular method exit
checker/tests/resourceleak/RequiredMethodNotCalledMessage.java:26: error: [required.method.not.called] Required method close may not have been invoked on resource or any of its aliases.
        DemoResource resource = new DemoResource();
                     ^
  The type of object is: RequiredMethodNotCalledMessage.DemoResource.
  Reason for going out of scope: possible exceptional exit before required method call, triggered by maybeThrows() throwing exception type java.io.IOException
checker/tests/resourceleak/RequiredMethodNotCalledMessage.java:37: error: [required.method.not.called] Required method close may not have been invoked on field field or any of its aliases.
            field = new DemoResource();
                  ^
  The type of object is: RequiredMethodNotCalledMessage.DemoResource.
  Reason for going out of scope: field assignment outside method or declaration might overwrite field's current value
checker/tests/resourceleak/RequiredMethodNotCalledMessage.java:43: error: [required.method.not.called] Required method close may not have been invoked on field field or any of its aliases.
            field = new DemoResource();
                  ^
  The type of object is: RequiredMethodNotCalledMessage.DemoResource.
  Reason for going out of scope: non-final owning field might be overwritten
4 errors

[wmdietl]

Thanks, that reads better than before.
@thisisalexandercook did you make sure the manual needs no updates?

[wmdietl]

@thisisalexandercook Please merge in master and move the fixed issue number to the next release.
Please check whether the error messages appear anywhere else in the repo, e.g. in the manual, and update them if needed.
Thanks!

[thisisalexandercook]

Only the error key appears in the manual so no updates should be needed.