os: improve Getwd documentation

1. Replace "rooted path name" with a more accurate "absolute path name".
Using "absolute" seems to be preferred terminology on Unix. On Windows,
there are multiple roots in Windows, thus "a rooted path name" can be
one relative to a drive letter or a server name. Note that Windows'
GetCurrentDirectory documentation [1] also says "the absolute path to the
current directory".

2. Add a note about using $PWD on Unix.

[1]: https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getcurrentdirectory

Change-Id: Ic310f0f8776ff059544789306ae5cfa1fa267b12
Reviewed-on: https://go-review.googlesource.com/c/go/+/607615


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Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
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