annotate method from @ccallable with @__doc__ (JuliaLang#51587)

This allows you to attach a docstring to a `@ccallable` method
definition.

Fixes JuliaLang#51586

Author: mbauman

base/c.jl

@@ -540,7 +540,7 @@ function expand_ccallable(rt, def)
                 end
             end
             return quote
-                $(esc(def))
+                @__doc__ $(esc(def))
                 _ccallable($(esc(rt)), $(Expr(:curly, :Tuple, esc(f), map(esc, at)...)))
             end
         end

test/docs.jl

@@ -1517,3 +1517,11 @@ struct S41727
 end
 @test S41727(1) isa S41727
 @test string(@repl S41727.x) == "x is 4\n"
+
+"ensure we can document ccallable functions"
+Base.@ccallable c51586_short()::Int = 2
+"ensure we can document ccallable functions"
+Base.@ccallable c51586_long()::Int = 3
+
+@test docstrings_equal(@doc(c51586_short()), doc"ensure we can document ccallable functions")
+@test docstrings_equal(@doc(c51586_long()), doc"ensure we can document ccallable functions")

test/llvmcall.jl

@@ -147,6 +147,10 @@ module ObjLoadTest
     using Base: llvmcall, @ccallable
     using Test
     didcall = false
+    """    jl_the_callback()
+
+    Sets the global didcall when it did the call
+    """
     @ccallable Cvoid function jl_the_callback()
         global didcall
         didcall = true