Convert the documentation to Asciidoc

A few small revisions were made, and Erlang.mk has been updated.

doc/src/guide/constraints.ezdoc

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-::: Constraints
-
-Cowboy provides an optional constraints based validation feature
-when interacting with user input.
-
-Constraints are first used during routing. The router uses
-constraints to more accurately match bound values, allowing
-to create routes where a segment is an integer for example,
-and rejecting the others.
-
-Constraints are also used when performing a match operation
-on input data, like the query string or cookies. There, a
-default value can also be provided for optional values.
-
-Finally, constraints can be used to not only validate input,
-but also convert said input into proper Erlang terms, all in
-one step.
-
-:: Structure
-
-Constraints are provided as a list of fields and for each
-field a list of constraints for that field can be provided.
-
-Fields are either the name of the field; the name and
-one or more constraints; or the name, one or more constraints
-and a default value.
-
-When no default value is provided then the field is required.
-Otherwise the default value is used.
-
-All constraints for a field will be used to match its value
-in the order they are given. If the value is modified by a
-constraint, the next constraint receives the updated value.
-
-:: Built-in constraints
-
-||	Constraint	Description
-|
-|	int			Convert binary value to integer
-|	nonempty	Ensures the binary value is non-empty
-
-:: Custom constraint
-
-In addition to the predefined constraints, Cowboy will accept
-a fun. This fun must accept one argument and return one of
-`true`, `{true, NewValue}` or `false`. The result indicates
-whether the value matches the constraint, and if it does it
-can optionally be modified. This allows converting the value
-to a more appropriate Erlang term.
-
-Note that constraint functions SHOULD be pure and MUST NOT crash.