docs: clarify mkdir does not throw with recursive: true (#33019)
The JSDoc for `Deno.mkdir` and `Deno.mkdirSync` said "Defaults to
throwing error if the directory already exists" without mentioning that
`recursive: true` suppresses this error.
The `MkdirOptions.recursive` property already documented this correctly,
but the function-level docs were misleading. Changed to "Throws if the
directory already exists, unless `recursive` is set to `true`."
Fixes #21768
