[Testing guide] Use "Fixtures API" instead of "fixture api".
[Testing guide] Using "Fixtures API" consistently

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.
Fix grammar on ActiveRecord::AttributeMethods doc.

[ci skip]
FIX: absolute_path may be nil for code added via instance_eval
Merge pull request #17860 from SamSaffron/fix_engine_backtrace

FIX: absolute_path may be nil for code added via instance_eval