Rollup merge of #76141 - jyn514:config.toml, r=Mark-Simulacrum

Address review comments about config.toml from rustc-dev-guide PR

This info was lost in #74334. See also rust-lang/rustc-dev-guide#795 (review).
r? @Mark-Simulacrum or @eddyb

Author: tmandry

config.toml.example

@@ -309,7 +309,9 @@
 # Number of codegen units to use for each compiler invocation. A value of 0
 # means "the number of cores on this machine", and 1+ is passed through to the
 # compiler.
-#codegen-units = 1
+#
+# Uses the rustc defaults: https://doc.rust-lang.org/rustc/codegen-options/index.html#codegen-units
+#codegen-units = if incremental { 256 } else { 16 }
 
 # Sets the number of codegen units to build the standard library with,
 # regardless of what the codegen-unit setting for the rest of the compiler is.
@@ -392,7 +394,7 @@
 # desired in distributions, for example.
 #rpath = true
 
-# Emits extra output from tests so test failures are debuggable just from logfiles.
+# Prints each test name as it is executed, to help debug issues in the test harness itself.
 #verbose-tests = false
 
 # Flag indicating whether tests are compiled with optimizations (the -O flag).