Add missing docs for ldap attribute configuration

User Story

As a SysAdmin, I will use this work when I want to customize the LDAP configuration to support LDAP installations that use email for a users mail attribute instead of mail.

Description

In Chef Server 12.15.0 a several new configuration option were added for ldap attribute fields but never added to the documentation.

Feature announcement here: https://github.com/chef/chef-server/blob/master/RELEASE_NOTES.md#user-customization-of-field-mapping

Here is the code with the various new configs that are available: https://github.com/chef/chef-server/blob/ac11df3d7c88b164a16beccdd18c12d403e87967/omnibus/files/private-chef-cookbooks/private-chef/templates/default/oc_erchef.config.erb#L102-L108

            %% LDAP Attribute Mappings
            {display_name_attribute, "<%= node['private_chef']['ldap']['display_name_attribute'] || 'displayname' %>" },
            {first_name_attribute,   "<%= node['private_chef']['ldap']['first_name_attribute']   || 'givenname' %>" },
            {last_name_attribute,    "<%= node['private_chef']['ldap']['last_name_attribute']    || 'sn' %>" },
            {common_name_attribute,  "<%= node['private_chef']['ldap']['common_name_attribute']  || 'cn' %>" },
            {country_attribute,      "<%= node['private_chef']['ldap']['country_attribute']      || 'c' %>" },
            {city_attribute,         "<%= node['private_chef']['ldap']['city_attribute']         || 'l' %>" },
            {email_attribute,        "<%= node['private_chef']['ldap']['email_attribute']        || 'mail' %>" },

The documentation should be added to:
https://docs.chef.io/config_rb_server_optional_settings.html#ldap
and
https://docs.chef.io/server_ldap.html

The warning at the under https://docs.chef.io/server_ldap.html#configure-ldap should also be updated to indicate the mail attribute is now configurable instead of being required.

Chef Version

>= 12.15.0

Platform Version

All supported by Chef-Server

Definition of Done

Demo Script / Repro Steps

Read the LDAP sections in config_rb_server_optional_settings.html#ldap and server_ldap.html, then compare them with the referenced oc_erchef.config.erb attribute mappings and the release announcement. Document the configurable LDAP attributes, including their defaults, in both locations and update the configure-ldap warning to reflect that the mail attribute is configurable.