message: update doc, fix rustc instructions
diff: @@ -22,15 +22,16 @@ python x.py --stage 1
  Run the compiler on a source file, supplying two additional debugging flags with `-Z`:
  ```
  -rustc -Z profile-queries -Z dump-dep-graph foo.rs
  +rustc -Z profile-queries -Z incremental=cache foo.rs
  ```
  Regarding the two additional parameters:
  - `-Z profile-queries` tells the compiler to run a separate thread
  - that profiles the queries made by the main compiler thread(s).
  -- `-Z dump-dep-graph` tells the compiler to "dump" various files that
  - describe the compilation dependencies.
  + that profiles the queries made by the main compiler thread(s). -
  + `-Z incremental=cache` tells the compiler to "cache" various files
  + that describe the compilation dependencies, in the subdirectory
  + `cache`.
  This command will generate the following files:
  