docs(datatype): add a note about BIGINT datatype conversion to string (#5796)

* docs(datatype): add a note about BIGINT datatype conversion to string

This helps users to understand the reason to this conversion and avoid more open issues to Sequelize like these:
- https://github.com/sequelize/sequelize/issues/1774
- https://github.com/sequelize/sequelize/issues/4550

* docs(datatype): add documentation to source code

docs/api/datatypes.md

@@ -89,6 +89,8 @@ Available properties: `UNSIGNED`, `ZEROFILL`
 
 A 64 bit integer.
 
+Note: an attribute defined as `BIGINT` will be treated like a `string` due this [feature from node-postgres](https://github.com/brianc/node-postgres/pull/353) to prevent precision loss. To have this attribute as a `number`, this is a possible [workaround](https://github.com/sequelize/sequelize/issues/2383#issuecomment-58006083).
+
 Available properties: `UNSIGNED`, `ZEROFILL`
 
 ***
@@ -362,4 +364,4 @@ A geography datatype represents two dimensional spacial objects in an elliptic c
 
 ***
 
-_This document is automatically generated based on source code comments. Please do not edit it directly, as your changes will be ignored. Please write on <a href="irc://irc.freenode.net/#sequelizejs">IRC</a>, open an issue or a create a pull request if you feel something can be improved. For help on how to write source code documentation see [JSDoc](http://usejsdoc.org) and [dox](https://github.com/tj/dox)_
\ No newline at end of file
+_This document is automatically generated based on source code comments. Please do not edit it directly, as your changes will be ignored. Please write on <a href="irc://irc.freenode.net/#sequelizejs">IRC</a>, open an issue or a create a pull request if you feel something can be improved. For help on how to write source code documentation see [JSDoc](http://usejsdoc.org) and [dox](https://github.com/tj/dox)_

lib/data-types.js

@@ -277,6 +277,8 @@ INTEGER.prototype.validate = function(value) {
 /**
  * A 64 bit integer.
  *
+ * Note: an attribute defined as `BIGINT` will be treated like a `string` due this [feature from node-postgres](https://github.com/brianc/node-postgres/pull/353) to prevent precision loss. To have this attribute as a `number`, this is a possible [workaround](https://github.com/sequelize/sequelize/issues/2383#issuecomment-58006083).
+ *
  * Available properties: `UNSIGNED`, `ZEROFILL`
  *
  * @property BIGINT