os, syscall: changes to os.Err* and syscall.Errno not mentioned in release notes and syscall package docs

[bcmills]

The changes made to the os and syscall package for #30322 are not mentioned in the Go 1.13 release notes, nor in the documentation for syscall.Errno or the syscall package.

Because users in general cannot redefine the error behavior of the errors defined by their dependencies, as a user I would not expect syscall.Errno errors to be defined as equivalent to errors defined in the higher-level os package. As noted in #32463 and #33411, the fact that they are has significant, non-trivial implications.

If errors.Is is now recommended and supported for checking the properties of syscall.Errno errors, the syscall.Errno documentation and the Go 1.13 release notes should be updated to mention that.

CC @neild @jba @mpvl @rsc @andybons @ianlancetaylor

[ianlancetaylor]

Ping @jba Want to write something about this as you've written other error-related docs?

[gopherbot]

Change https://golang.org/cl/191337 mentions this issue: syscall: document relationship among Errno, errors.Is and os.Err*