Docs: use doxygen formatting for DNA

Differentiate doc-strings from title/section text.
Also use explicit doxygen references to struct members
so it's not ambiguous which member is being referenced.

Note that these changes aren't complete (some files weren't touched).

Docs: use doxygen formatting for BLI

Differentiate doc-strings from title/section text.

Docs: minor correction to doc-string

The text editor no longer accumulates changes.

UI: move "undo history" from a custom popup to a menu type

This lets the undo history expand as a regular sub-menu
instead of being a popup.

Also disable the active undo step menu item as this is a no-op.

Cleanup: spelling in comments

Cleanup: use more common naming suffix for item callbacks

Fix T94115: Selecting current action in undo history undoes all

When selecting the current undo step there is no need to do anything.

Fix and minor refactor to de-duplicate refreshing after running
undo/redo & undo history.

Docs: add doc-strings for BLI_path functions

Cleanup: simplify file saving logic

Revert part of the fix from 073669dd8588a3b80dfffee98b4f239b4baee8c8
that initialized the file-path on first save as it's no longer needed.

Also remove relbase argument to BLI_path_normalize as the destination
file paths shouldn't use relative locations.

WM: various changes to file writing behavior

Saving with only a filename (from Python) wasn't being prevented,
while it would successfully write the file to the working-directory,
path remapping and setting relative paths wouldn't work afterwards
as `Main.filepath` would have no directory component.

Disallow this since it's a corner case which only ever occurs
when path names without any directories are used from Python,
the overhead of expanding the working-directory for all data saving
operations isn't worthwhile.

The following changes have been made:

- bpy.ops.wm.save_mainfile() without a filepath argument
fails & reports and error when the file hasn't been saved.

Previously it would write to "untitled.blend" and set the
`G.main->filepath` to this as well.

- bpy.ops.wm.save_mainfile(filepath="untitled.blend")
fails & reports and error as the filename has no directory component.

- `BLI_path_is_abs_from_cwd` was added to check if the path would
attempt to expand to the CWD.