Remove `-a/--app` from Quickstart documentation (#2046)

* Remove `-a/--app` from Quickstart documentation

As mentioned in #2009, simplifying the CLI saw the removal of the `-a/--app` flag. Therefore, the only way to specify the module to import is by setting `FLASK_APP`.

* Remove misleading `either` from CLI help

The CLI help details how to run the application, but still uses the phrasing "either through the `FLASK_APP`...". This likely is an artifact from when `-a/--app` was still present in the CLI.

docs/quickstart.rst

@@ -102,10 +102,10 @@ docs to see the alternative method for running a server.
 Invalid Import Name
 ```````````````````
 
-The ``-a`` argument to :command:`flask` is the name of the module to
-import.  In case that module is incorrectly named you will get an import
-error upon start (or if debug is enabled when you navigate to the
-application).  It will tell you what it tried to import and why it failed.
+The ``FLASK_APP`` environment variable is the name of the module to import at 
+:command:`flask run`. In case that module is incorrectly named you will get an 
+import error upon start (or if debug is enabled when you navigate to the 
+application). It will tell you what it tried to import and why it failed.
 
 The most common reason is a typo or because you did not actually create an
 ``app`` object.

flask/cli.py

@@ -469,7 +469,7 @@ def shell_command():
 cli = FlaskGroup(help="""\
 This shell command acts as general utility script for Flask applications.
 
-It loads the application configured (either through the FLASK_APP environment
+It loads the application configured (through the FLASK_APP environment
 variable) and then provides commands either provided by the application or
 Flask itself.