encoding/pem: add Encode, EncodeToMemory docs

encoding/pem: add Encode, EncodeToMemory docs

Included in a warning that EncodeToMemory may return an incomplete PEM
encoded structure if invalid headers are supplied. Example:

	pem.EncodeToMemory(&pem.Block{
		Headers: map[string]string{
			"a":   "test1",
			"b:c": "test2",
		},
	})

Returns:

	-----BEGIN -----
	a: test1

Change-Id: Ia9cf0202f985e3cf210aabb6f07667e581ff081f
Reviewed-on: https://go-review.googlesource.com/77790
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>