This patch modifies the GenMarkdownCustom to skip writing a Synopsis
header with the `cmd.Short` duplicated both before and after the header.
Instead, it only writes the `### Synopsis` header and the paragraph when
a `cmd.Long` has some kind of content in it.
Adds `TestGenMdDocWithNoLongOrSynopsis` as the test case.
Signed-off-by: Marc Lopez <marc5.12@outlook.com>
