 Layer "lang#sh" does not document bash_file_head as an attribute that is customisable through init.toml
## Expected behavior

Similar to python_file_head in the lang#python layer, the attribute bash_file_head must be documented so that users apprehend its usage and purpose.

## The reproduce ways from Vim starting (Required!)
Not applicable 

## Output of the `:SPDebugInfo!`
Not required

## Screenshots

None
