add documentation for Result Location Semantics

[andrewrk]

• how result locations flow across expressions
  • variables
  • struct initialization
  • array initialization
  • union initialization
  • if
  • while
  • switch
  • return
  • function calls - return values & parameters
  • catch
  • orelse
  • @bitCast
• mention result locations: unwrap optional and error unions so that the payload can be non-copied #2761 and result location: ability to refer to the return result location before the return statement #2765

[vi]

What is a Result Location?

[fengb]

Here’s the original issue: #287

My best summary: when a function returns a newly created value, it will automatically put it into the function’s return location instead of making a copy. Previously, this was only a potential optimization from LLVM, but now it’s guaranteed by Zig.