docs: Clarify obs_source_update can accept NULL for settings argument

[norihiro]

Description

This PR adds the parameter description of the API obs_source_update.

Also fixed a gramatical error; will be not be called --> will not be called.

Motivation and Context

In the implementation, settings can be NULL and it will be just ignored if it is NULL.

obs-studio/libobs/obs-source.c

Lines 990 to 992 in 15e9242

	if (settings) {
	obs_data_apply(source->context.settings, settings);
	}

Passing NULL as the settings argument is useful to trigger update callback.

I realized AJA source calls obs_source_update with the existing settings. This is redundant, we can just pass NULL.
https://github.com/obsproject/obs-studio/blob/master/plugins/aja/aja-source.cpp#L327

How Has This Been Tested?

OS: Fedora 39

Called obs_source_update with the NULL argument and confirmed the settings was not changed.

Types of changes

• Documentation (a change to documentation pages)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

Sidenote:

When passing a valid obs_data_t object, elements that exists in the current settings but does not exist in the passed object are not removed.
This is because obs_source_update traverse each element in the given obs_data_t object and apply it to the current settings.
It would be good for the document to describe this behavior. However, I don't have a good idea to phrase this in concise sentence.

[gxalpha]

I would argue that if you’re calling this with NULL, you’re probably doing something in unintended ways. Personally I hate that it’s even possible to update source settings without calling obs_source_update (e.g. by directly changing the settings returned via obs_source_settings).

[norihiro]

We have already had a usage in the image slideshow, which is recently added by Lain.

obs-studio/plugins/image-source/obs-slideshow-mk2.c

Line 843 in 15e9242

obs_source_update(source, NULL);