Improve the descriptions of ipv[46].method in the man page

The man page entries for ipv[46].method are not clear. For a better user experience, these man page entries should be improved.

1. Remove the mention of NMSettingIP4Config in the second paragraph of the ipv6.method description.

2. Remove the mention of NMSettingIP6Config in the second paragraph of the ipv4.method description.

3. It's not clear what See the subclass-specific documentation for other values. means and where the reader can find this information. There are only 5 values for IPv4 and 7 values for IPv6. Can they be briefly described in *.method instead of redirecting the reader?

4. The meaning of the stand-alone paragraph For methods that imply no upstream network, such as "shared" or "link-local", these properties must be empty. is not clear. If this information is related to the previous paragraph (about ignore-auto-*), then it should be moved there.

5. The same description of the shared value appears in both ipv4.method and ipv6.method. The latter should have it's own description and mention the range that is used when ipv6.method=shared is set.