document what each tools does

src/bootstrap/lib.rs

@@ -200,7 +200,11 @@ use cache::{Interned, INTERNER};
 use toolstate::ToolState;
 
 const LLVM_TOOLS: &[&str] = &[
-    "llvm-nm", "llvm-objcopy", "llvm-objdump", "llvm-profdata", "llvm-size",
+    "llvm-nm", // used to inspect binaries; it shows symbol names, their sizes and visibility
+    "llvm-objcopy", // used to transform ELFs into binary format which flashing tools consume
+    "llvm-objdump", // used to disassemble programs
+    "llvm-profdata", // used to inspect and merge files generated by profiles
+    "llvm-size", // prints the size of the linker sections of a program
 ];
 
 /// A structure representing a Rust compiler.