Session settings docs lack aliases

[rschu1ze]

There are two sub-problems:

1. The docs for system table system.settings don't mention field alias_for. (Funnily, this isn't the case for the example section).

2. The docs of ClickHouse's session settings also don't mention setting aliases. For example, setting allow_experimental_time_time64_type is documented but setting enable_time_time64_type is not (the latter is an alias of the former). This is unfortunate because the alias mechanism is often used to promote experimental features to GA (like in the previous example), and users won't be able to find the newer setting names.