os/exec: document a method to check if a process is alive

[ferrmin]

Fixes golang#34396

Change-Id: I35c4e3447f84e349adf7edba92ccb19b324bfe14
GitHub-Last-Rev: 4f06764
GitHub-Pull-Request: golang#60763
Reviewed-on: https://go-review.googlesource.com/c/go/+/502815
TryBot-Result: Gopher Robot gobot@golang.org
Reviewed-by: Dmitri Shuralyov dmitshur@google.com
Run-TryBot: Ian Lance Taylor iant@golang.org
Reviewed-by: Ian Lance Taylor iant@google.com
Run-TryBot: Ian Lance Taylor iant@google.com
Auto-Submit: Ian Lance Taylor iant@google.com

This PR will be imported into Gerrit with the title and first
comment (this text) used to generate the subject and body of
the Gerrit change.

Please ensure you adhere to every item in this list.

More info can be found at https://github.com/golang/go/wiki/CommitMessage

• The PR title is formatted as follows: net/http: frob the quux before blarfing
  • The package name goes before the colon
  • The part after the colon uses the verb tense + phrase that completes the blank in,
    "This change modifies Go to ___________"
  • Lowercase verb after the colon
  • No trailing period
  • Keep the title as short as possible. ideally under 76 characters or shorter
• No Markdown
• The first PR comment (this one) is wrapped at 76 characters, unless it's
  really needed (ASCII art, table, or long link)
• If there is a corresponding issue, add either Fixes #1234 or Updates #1234
  (the latter if this is not a complete fix) to this comment
• If referring to a repo other than golang/go you can use the
  owner/repo#issue_number syntax: Fixes golang/tools#1234
• We do not use Signed-off-by lines in Go. Please don't add them.
  Our Gerrit server & GitHub bots enforce CLA compliance instead.
• Delete these instructions once you have read and applied them