Consider this example:

Solve[expr,vars] attempts to solve the system expr of equations or inequalities for the variables vars.

Where can I look to find out what expr or var refers to?

Entering expr into the language search returns nothing particularly relevant to what is allowed for expr" nor is expr linked to a definition. Moreover, even if one (quite reasonably) imputes to expr the meaning "expression" and enters "expression" into the language search, one is taken to a list of results that don't seem to get to the point of answering my question about what is permitted in 'Solve' as an expr.

For example, can 'Solve' make sense when an expr is something other than a set of relations (==, !=, etc.), like:

Solve[{x+x,x*2},x]

Note, I'm aware that I can merely enter this into the interpreter to get the error informing me that there is something to be desired in the documentation in this particular case. But I'm not here interested in reporting that particular difficulty but, rather, in finding out how I can look up the meaning of these syntactic (non)terminals even assuming the documentation is completely correct.