m.get_login_token capability is not documented

[hughns]

Link to problem area:
https://spec.matrix.org/v1.7/client-server-api/#get_matrixclientv3capabilities

Issue
https://spec.matrix.org/v1.7/client-server-api/#post_matrixclientv1loginget_token makes reference to a m.get_login_token capability. However, the capability section 5 does not describe the capability: https://spec.matrix.org/v1.7/client-server-api/#get_matrixclientv3capabilities

Expected behaviour

The m.get_login_capability should be described in section 5.

[richvdh]

for archaeology: the new endpoint was added in #1530, which appear to have forgotten to document the capability in section 5.

When fixing this, don't forget to add it to the table at https://spec.matrix.org/v1.7/client-server-api/#_matrixclientv3capabilities_capabilities.