Update docs: now more than just hollowing functions

book/src/welcome.md

@@ -1,9 +1,8 @@
 # Welcome to cargo-mutants
 
 cargo-mutants is a mutation testing tool for Rust. It helps you improve your
-program's quality by finding functions whose body could be replaced without
-causing any tests to fail. Each such case indicates, perhaps, a gap in semantic
-code coverage by your tests, where a bug might be lurking.
+program's quality by finding places where bugs can be inserted
+causing any tests to fail.
 
 **The goal of cargo-mutants is to be _easy_ to run on any Rust source tree, and
 to tell you something _interesting_ about areas where bugs might be lurking or