docs/ src/__init__.py: improve docs for insert_textbox() methods.

[julian-smith-artifex-com]

[This was prompted by someone on #pymupdf who pointed out that our docs don't agree with our code for Page/Shape.insert_textbox().]

src/init.py:
In Page.insert_textbox() and Shape.insert_textbox(), sort args to simplify
comparison.

docs/shape.rst::insert_textbox():
Use sorted multiple lines for args to simplify comparison.

docs/page.rst:insert_textbox():
Use sorted multiple lines for args to simplify comparison.

Describe <overlay> explicitly because not in Shape.insert_textbox().

Fixed args to match the code:
* Removed <charwidths>.
* Removed <idx>.
* Added <lineheight>.
* Added <set_simple>.