Add docstring for whichtt

Author: timholy

docs/src/dev_reference.md

@@ -1,6 +1,6 @@
 # Function reference
 
-## Top-level
+## Running the interpreter
 
 ```@docs
 @interpret
@@ -93,4 +93,5 @@ JuliaInterpreter.linenumber
 JuliaInterpreter.statementnumber
 JuliaInterpreter.Variable
 JuliaInterpreter.locals
+JuliaInterpreter.whichtt
 ```

src/utils.jl

@@ -25,6 +25,11 @@ function to_function(@nospecialize(x))
     isa(x, GlobalRef) ? getfield(x.mod, x.name) : x
 end
 
+"""
+    method = whichtt(tt)
+
+Like `which` except it operates on the complete tuple-type `tt`.
+"""
 function whichtt(@nospecialize(tt))
     m = ccall(:jl_gf_invoke_lookup, Any, (Any, UInt), tt, typemax(UInt))
     m === nothing && return nothing