Add explanation to the "programming languages used" chart

What does this MR do?

The chart showing "Programming languages used in this repository" did not explain its methodology. That could be confusing, because the methodology is actually fairly sophisticated (for example, it ignores vendored code). This adds two sentences to explain, based on the github-linguist documentation.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #17800 (closed)