Clarify min, max and rank documentation

[Marcono1234]

Currently the documentation sounds like the <expression> must always be "of numeric type or of type string", however that is incorrect, as also described correctly by the full language specification:

The type of the expression in a max, min or rank aggregation without an ordering directive expression must be an orderable type

(emphasis mine)

For example the following is valid:

import java

select min(File f | f.getExtension() = "java" | f order by f.getTotalNumberOfLines())

import java

select rank[5](File f | f.getExtension() = "java" | f order by f.getTotalNumberOfLines())

[aschackmull]

LGTM, thanks!