docs: Style Guide's "Directive selectors" doesn't mention prefixing selectors

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[x] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

In the Directive selectors section of the Style Guide, nothing is mentioned about prefixing selectors for name spacing.

However, in the Guide for Write a structural directive, in the second to last paragraph, you can find the following:

The directive attribute name should be spelled in lowerCamelCase and begin with a prefix. Don’t use ng. That prefix belongs to Angular. Pick something short that fits you or your company. In this example, the prefix is app.

This is not mentioned in the style guide. Worse, Angular’s directives seem to only be prefixed with ng when it’s a “single word” directive—ngForm—but they are not prefixed with they are multiple words—formControlName.

Expected behavior

The Style Guide should make a recommendation about prefixing or not prefixing directive attribute names. I would be more than happy to submit a PR to clarify the issue in the Style Guide once I receive direction. 👍

[petebacondarwin]

I have to admit I am not sure. I would imagine that a number people (and maybe tools) link through to these styles, so perhaps the numbering should stay stable.

[shaungrady]

So if I’m understanding correctly, this is how it should be reorganized:

Naming

• …
• Component selectors
• Component custom prefix
• Directive selectors
• Directive custom prefix
• …

…

Components

• ~Component selector names~
• …

In Naming, I think it makes sense for the Component section to appear before the Directive section as components are more common. Also, renaming Custom prefix for components to Component custom prefix keeps it visually connected to the preceding Component selectors.

If that sounds good I’ll submit a PR with those changes. 😄