Missing (or difficult-to-find) documentation for `eq`, `gt`, etc.

[trombonehero]

Diesel provides lots of DSL sugar, and from the examples I can figure out how to do things like:

users.filter(created.gt(/* ... */))

However, I can't find a list of eq, gt, etc., in the documentation, so I don't know what predicates are supported. For example, there's probably a way to write a "non-null" predicate, but I can't find it in the documentation. Maybe it does exist, but if so, some indexing may be required...

[sgrif]

http://docs.diesel.rs/diesel/expression/expression_methods/global_expression_methods/trait.ExpressionMethods.html#method.eq

The docs have a search function, that method is the first result when searching for eq. The method for IS NOT NULL is called is_not_null, and is documented here (again, found by typing is_not_null into the search bar)

[trombonehero]

It's true that is_not_null is documented, but to find that documentation I need to know either:

1. that it's called is_not_null (which is the thing I wanted to learn) or
2. that the list of such expressions are documented in diesel::expression::expression_methods::global_expression_methods::ExpressionMethods.

My suggestion is that something between the high-level http://diesel.rs/guides/getting-started and the low-level API docs could be helpful for people trying to learn Diesel. Something like "here's how these SQL clauses translate into Diesel expressions" would be great.

[sgrif]

Yes, writing a guide on this is the second one on the list that I give for people who reach out to help with the docs.