Document destructuring syntax in anonymous function argument (#39024)

When destructuring a single tuple in an anonymous function argument,
you have to include an extra comma to force a 1-tuple. Otherwise ((x,y))
collapses to (x,y). This is not the case for named functions.

Co-authored-by: Graham Smith <grahamas@gmail.com>

Author: Keno

doc/src/manual/functions.md

@@ -395,6 +395,15 @@ julia> gap(minmax(10, 2))
 Notice the extra set of parentheses in the definition of `gap`. Without those, `gap`
 would be a two-argument function, and this example would not work.
 
+For anonymous functions, destructuring a single tuple requires an extra comma:
+
+```
+julia> map(((x,y),) -> x + y, [(1,2), (3,4)])
+2-element Array{Int64,1}:
+ 3
+ 7
+```
+
 ## Varargs Functions
 
 It is often convenient to be able to write functions taking an arbitrary number of arguments.
@@ -681,8 +690,8 @@ end
 ```
 
 The `do x` syntax creates an anonymous function with argument `x` and passes it as the first argument
-to [`map`](@ref). Similarly, `do a,b` would create a two-argument anonymous function, and a
-plain `do` would declare that what follows is an anonymous function of the form `() -> ...`.
+to [`map`](@ref). Similarly, `do a,b` would create a two-argument anonymous function. Note that `do (a,b)` would create a one-argument anonymous function,
+whose argument is a tuple to be deconstructed. A plain `do` would declare that what follows is an anonymous function of the form `() -> ...`.
 
 How these arguments are initialized depends on the "outer" function; here, [`map`](@ref) will
 sequentially set `x` to `A`, `B`, `C`, calling the anonymous function on each, just as would happen