Manual: anonymous function in pipeline needs parentheses (JuliaLang#43661)

Consider:
```julia
d = (1:10 
    .|> x -> x^2
    |> sum
)
```
This seems to compute a sum of squares.
But it does not. It outputs a vector of squares. (Because `|> sum` is parsed as being part of the anonymous function's body).
Parentheses are needed for the desired (expected?) behaviour:
```julia
d = (1:10 
    .|> (x -> x^2)
    |> sum
)
```

This PR adds a small note and example to the "Function composition and piping" section of the manual to flag this.

As an aside, this confusion is even present in the [docstring](https://docs.julialang.org/en/v1/base/base/#Base.:|%3E) for `|>`: 
> Applies a function to the preceding argument. This allows for easy function chaining.
> ```julia
[1:5;] |> x->x.^2 |> sum |> inv
```
This is parsed as
```julia
[1:5;] |> x-> (x.^2 |> sum |> inv)
```
but it feels like the author meant
```julia
[1:5;] |> (x-> x.^2) |> sum |> inv)
```.

Author: tfiers

doc/src/manual/functions.md

@@ -912,7 +912,7 @@ julia> (sqrt ∘ sum)(1:10)
 7.416198487095663
 ```
 
-The pipe operator can also be used with broadcasting, as `.|>`, to provide a useful combination of the chaining/piping and dot vectorization syntax (described next).
+The pipe operator can also be used with broadcasting, as `.|>`, to provide a useful combination of the chaining/piping and dot vectorization syntax (described below).
 
 ```jldoctest
 julia> ["a", "list", "of", "strings"] .|> [uppercase, reverse, titlecase, length]
@@ -923,6 +923,19 @@ julia> ["a", "list", "of", "strings"] .|> [uppercase, reverse, titlecase, length
  7
 ```
 
+When combining pipes with anonymous functions, parentheses must be used if subsequent pipes are not to parsed as part of the anonymous function's body. Compare:
+
+```jldoctest
+julia> 1:3 .|> (x -> x^2) |> sum |> sqrt
+3.7416573867739413
+
+julia> 1:3 .|> x -> x^2 |> sum |> sqrt
+3-element Vector{Float64}:
+ 1.0
+ 2.0
+ 3.0
+```
+
 ## [Dot Syntax for Vectorizing Functions](@id man-vectorized)
 
 In technical-computing languages, it is common to have "vectorized" versions of functions, which