Update to help text

[Yegappan Lakshmanan]

Hi,

When adding the Vim9 argument type checks to the built-in functions,
I noticed some minor issues in the help text for these functions.

1. The help for some functions that accept a String argument have the
description that states that the argument is a string. But for some
functions, this is not called out.
2. Functions that accept a buffer number as an argument, the argument
type is not clearly called out.

I am attaching an update to the help to address this.

- Yegappan

[Bram Moolenaar]

Thanks, I'll include it.

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