docs: clarify required argument behavior in arguments guide

[mitre88]

Summary

• updates docs/arguments.rst to improve clarity and grammar
• aligns the “required arguments” note with the actual click.Argument behavior in src/click/core.py
• keeps scope to a single file, as requested in Improve documentation #3175

What changed

• clarified that arguments are required by default when nargs > 0 and no default is set
• documented how to make arguments optional (default or required=False)
• improved wording in a few related sections for readability and consistency

fixes #3175

[davidism]

Appears LLM generated. Removes important context.