crypto/x509: MarshalPKCS8PrivateKey doc says RSA private key while it supports more than that [1.14 backport]

[gopherbot]

@katiehockman requested issue #36735 to be considered for backport to the next 1.14 minor release.

@gopherbot please backport to 1.13 and 1.14

[gopherbot]

Change https://golang.org/cl/217999 mentions this issue: [release-branch.go1.14] crypto/x509: fix godoc for MarshalPKCS8PrivateKey

[gopherbot]

Closed by merging 3f0cded to release-branch.go1.14.

[dmitshur]

This change does not need to go through backport process because we're still in code freeze for 1.14, so all changes to master are expected to be included in subsequent release candidates (if any) and the final 1.14 release. Even if this commit wasn't explicitly cherry-picked to release-branch.go1.14, it would've been included via a merge commit such as CL 218318.

I'll update the CherryPickCandidate label to CherryPickApproved to reflect the reality.