Fix recently extended delete-indentation behavior

Fix recently extended delete-indentation behavior

* doc/lispref/text.texi (User-Level Deletion): Document new optional
arguments of delete-indentation.

* lisp/simple.el (delete-indentation): Do not barf if called
interactively when region is inactive. (bug#35021)
Do not skip blank lines. (bug#35036)
Consistently deactivate mark even when no text was changed.
Handle active region spanning a single line.

* test/lisp/simple-tests.el (simple-test--buffer-substrings):
New convenience function.
(simple-test--dummy-buffer, simple-test--transpositions): Use it.
(simple-delete-indentation-no-region)
(simple-delete-indentation-inactive-region): Update commentary.
Call delete-indentation interactively when testing for behavior with
inactive region and region is not explicitly defined.
(simple-delete-indentation-blank-line)
(simple-delete-indentation-boundaries)
(simple-delete-indentation-region)
(simple-delete-indentation-prefix): New tests.