Update QA docs: clarify spec vs. specs/features and use end-to-end instead of integration

What does this MR do?

I and almost every candidate I've interviewed has been at least briefly confused upon finding spec files in qa/spec/ as well as qa/qa/specs/ (not to mention spec/).

This MR notes the difference.

It also replaces instances of integration with end-to-end in qa/README.md because the later is what we more commonly use to refer to the QA tests.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards