crypto/x509: reformat template members in docs

crypto/x509: reformat template members in docs

It's easier to skim a list of items visually when the
items are each on a separate line. Separate lines also
help reduce diff size when items are added/removed.

The list is indented so that it's displayed preformatted
in HTML output as godoc doesn't support formatting lists
natively yet (see #7873).

Change-Id: Ibf9e92437e4b464ba58ea3ccef579e8df4745d75
Reviewed-on: https://go-review.googlesource.com/113915
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>