Document cached config behavior

[kpicaza]

Is your feature request related to a problem? Please describe.

Current documentation does not remark the behavior of the cached config, this file is auto-generated in the first request if not exist, then it will be loaded from the filesystem instead of parsing again entire configs set.

This behavior generates confusion on development because if you add a new dependency via config the framework should no be able to load it until the cached config is regenerated.

To avoid this issue we use the development mode, this consist of avoiding to generate cached config file and regenerating every config on each request, to activate it the developer can use the dev-tool command bin/console config:development-mode or manually deleting cached config files and copying the config/services/*.dev.{php,yaml}.dist to the same path but without .dist` extension.