[DOCS] Update documentation for SAML metadata signing  (elastic/x-pack-elasticsearch#4356)
[DOCS] Update documentation for SAML metadata signing  (elastic/x-pack-elasticsearch#4356)

Updates documentation describing the parameters for optionally signing the
SAML medatata document that the saml-metadata command creates.

Original commit: elastic/x-pack-elasticsearch@100102962d92364cd1ca4b0c2d2a04b27115fba3

strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra