[docs] Clarify the sense of --compile-command

In retrospect, it seems "obvious" that the sense of the return code is
the same as if it crashed on "interesting" inputs. But that didn't stop
me from spending more time than I care to admit verifying this.

llvm-svn: 264119

docs/CommandGuide/bugpoint.rst

@@ -151,7 +151,12 @@ OPTIONS
 **--compile-command** *command*
 
  This option defines the command to use with the **--compile-custom**
- option to compile the bitcode testcase. This can be useful for
+ option to compile the bitcode testcase. The command should exit with a
+ failure exit code if the file is "interesting" and should exit with a
+ success exit code (i.e. 0) otherwise (this is the same as if it crashed on
+ "interesting" inputs).
+
+ This can be useful for
  testing compiler output without running any link or execute stages. To
  generate a reduced unit test, you may add CHECK directives to the
  testcase and pass the name of an executable compile-command script in this form: