Add a module docstring

Author: timholy

src/CodeTracking.jl

@@ -1,3 +1,13 @@
+"""
+CodeTracking can be thought of as an extension of InteractiveUtils, and pairs well with Revise.jl.
+
+- `code_string`, `@code_string`: fetch the source code (as a string) for a method definition
+- `code_expr`, `@code_expr`: fetch the expression for a method definition
+- `definition`: a lower-level variant of the above
+- `pkgfiles`: return information about the source files that define a package
+- `whereis`: Return location information about methods (with Revise, it updates as you edit files)
+- `signatures_at`: return the signatures of all methods whose definition spans the specified location
+"""
 module CodeTracking
 
 using Base: PkgId
@@ -159,7 +169,7 @@ function signatures_at(filename::AbstractString, line::Integer)
             end
         end
     end
-    error("$filename not found in internal data, perhaps the package is not loaded (or not loaded with `includet`)")
+    throw(ArgumentError("$filename not found in internal data, perhaps the package is not loaded (or not loaded with `includet`)"))
 end
 
 """