doc: add section in fs to document mode argument

[trivikr]

• Version: master
• Platform: N/A
• Subsystem: fs

I noticed this while reviewing #20042
The description of mode argument is repeated in fs.access() and fs.accessSync(), and can be moved to a single location.

WDYT @nodejs/fs? This would be a good first issue for a new contributor.

[musgravejw]

Aren't these constants defined here?

Perhaps the documentation could link to the existing constants list.

[trivikr]

Yup.

But the following values will have to be added to the table:

• F_OK - This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
• X_OK - This has no effect on Windows (will behave like F_OK)

[shobhitchittora]

@trivikr I'm on it.