docs: Add doc comments to `std.math.isNan`

[sorairolake]

std.math.isNan has no doc comments, so add it.

[nektro]

what is this adding over the name of the function?

[sorairolake]

This function returns whether the given value is a NaN. This pull request adds documentation explaining this behavior. We can guess what this function does from the function name and examples, but I think it would be easier to understand if there is documentation.

[saurabh-mish]

This pull request adds documentation explaining this behavior. We can guess what this function does from the function name and examples, but I think it would be easier to understand if there is documentation.

Something along the lines of

/// Returns true if x is not a number, and false otherwise

would be better because NaN is not being used to describe the isNan function.

[sorairolake]

@saurabh-mish suggestion:

/// Returns `true` if `x` is not a number (NaN), and `false` otherwise.

or

/// Returns `true` if `x` is NaN (not a number), and `false` otherwise.