Rollup merge of #124282 - RalfJung:fill_utf16_buf, r=ChrisDenton

windows fill_utf16_buf: explain the expected return value

The comment just says "return what the syscall returns", but that doesn't work for all syscalls as the Windows API is not consistent in how buffer size is negotiated. For instance, GetUserProfileDirectoryW works a bit differently, and so home_dir_crt has to translate this to the usual protocol itself. So it's worth describing that protocol.

r? ``@ChrisDenton``