crypto/x509: MarshalPKCS8PrivateKey doc says RSA private key while it supports more than that [1.13 backport]

[gopherbot]

@katiehockman requested issue #36735 to be considered for backport to the next 1.13 minor release.

@gopherbot please backport to 1.13 and 1.14

[gopherbot]

Change https://golang.org/cl/217998 mentions this issue: [release-branch.go1.13] crypto/x509: fix godoc for MarshalPKCS8PrivateKey

[gopherbot]

Closed by merging 17ac3f6 to release-branch.go1.13.

[dmitshur]

This backport has been approved because it is a documentation fix.