unix ExitStatus: Clarify docs for .signal()
unix ExitStatus: Clarify docs for .signal()

We need to be clear that this never returns WSTOPSIG.  That is, if
WIFSTOPPED, the return value is None.

Signed-off-by: Ian Jackson <d68d4287c1430ca128ad85033356c9d259238c74@chiark.greenend.org.uk>

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust