Adder and remover sidenote

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

There are cases when you only want either adder or remover, important to know both of them need to be found if any is to work.
Merging emphasized notes

Merged both notes into one, in accordance to discussion feedback.
Update form_customization.rst

In the Form chapter of Book for Symphony 2.3, we use '{{ form(form) }}' to render the entire form ( in contrast to '{{ form_widget(form) }}' for older version of Symfony ). so maybe it should use '{{ form(form) }}' here for consistency.
Applied suggestion by Ryan
Update slash_in_parameter.rst

"name" in requirements is confusing, I had problem for doing the same and then after 2 days I found that the "name" should be same as the parameter, it is not part of symfony config

   requirements: 
        name: .+
Changed userName to username
describe how to access form errors
[Form] document $deep and $flatten of getErrors()
Completely re-reading the controller chapter
Removed unnecessary use statement in code example in the debugging cookbook
tweaks to the Twig reference
refer to the VarDumper component for dump()
Bot fixes
Fixed wrong indentation
remove versionadded directives for old versions
Renamed empty_value to placeholder
Documented html5 option
Documented expressionlanguage extensibility
Documented new tags
link to a bundle's current (not master) docs