wip: comments, symbols, metadata

We need a general explanation of what metadata actually is, how it is assigned and what it can be used for.

Example:
```
Metadata are parameters
that can be assigned to certain text sections
using `scope selectors`_.

These parameters can for example be used for:

- specifying the current comment syntax,
  even with embedded source code,
  so that you can toggle comments in any syntax,
- defining indentation rules for auto-indenting source code,
- selecting symbols that ST will allow you to browse to quickly (x-ref goto anything).

Furthermore, snippets have access to metadata specified in the ``shellVariables`` setting
which allows you to create a snippet
that has different contents
depending on where it's used.
```

Another thing I noticed: We should mention that syntax `includes` that cross-refernce other syntax files do NOT add its base scope name to the scope. So, for example with HTML including PHP,
```yaml
- name: source.php
  include: source.php
```
Otherwise the concept of metadata wouldn't work.