Document LdapGroupsMapping keystore password properties

A few properties are not documented.
hadoop.security.group.mapping.ldap.ssl.keystore.password
This property is used as an alias to get password from credential providers, or, fall back to using the value as password in clear text. There is also a caveat that credential providers can not be a HDFS-based file system, as mentioned in HADOOP-11934, to prevent cyclic dependency issue.
This should be documented in core-default.xml and GroupsMapping.md

hadoop.security.credential.clear-text-fallback
This property controls whether or not to fall back to storing credential password as cleartext.
This should be documented in core-default.xml.

hadoop.security.credential.provider.path
This is mentioned in CredentialProvider API Guide, but not in core-default.xml

The "Supported Features" in CredentialProvider API Guide should link back to GroupsMapping.md#LDAP Groups Mapping

hadoop.security.credstore.java-keystore-provider.password-file
This is the password file to protect credential files.