Add clarification to required files in About component file structure page

[multipasko]

General issue

Description:

https://devdocs.magento.com/guides/v2.3/extension-dev-guide/prepare/prepare_file-str.html

you say:

"All components require these three files:"

• etc/module.xml

This is not true. Only modules have this etc/module.xml file. Themes or language packages do not have this. I also believe that composer.json is not required but recommended to use.

Possible solutions:

Additional information:

[shrielenee]

Add more details to individual line items in Required files section.

Or "Components may require these three files:"

[atwixfirster]

Add more details to individual line items in Required files section.

Or "Components may require these three files:"

@shrielenee

A problem is a bit deeper than a simple description change :)

More details here: #6184

Thank you!