Mention .+= in response to why += allocates (JuliaLang#42745)

Co-authored-by: Simeon Schaub <[email protected]>

Co-authored-by: Simeon Schaub <[email protected]>

Author: LilithHafner

doc/src/manual/faq.md

@@ -834,9 +834,10 @@ no values and no subtypes (except itself). You will generally not need to use th
 
 ### Why does `x += y` allocate memory when `x` and `y` are arrays?
 
-In Julia, `x += y` gets replaced during parsing by `x = x + y`. For arrays, this has the consequence
+In Julia, `x += y` gets replaced during lowering by `x = x + y`. For arrays, this has the consequence
 that, rather than storing the result in the same location in memory as `x`, it allocates a new
-array to store the result.
+array to store the result. If you prefer to mutate `x`, use `x .+= y` to update each element
+individually.
 
 While this behavior might surprise some, the choice is deliberate. The main reason is the presence
 of immutable objects within Julia, which cannot change their value once created.  Indeed, a
@@ -869,8 +870,8 @@ After a call like `x = 5; y = power_by_squaring(x, 4)`, you would get the expect
     `x`, after the call you'd have (in general) `y != x`, but for mutable `x` you'd have `y == x`.
 
 Because supporting generic programming is deemed more important than potential performance optimizations
-that can be achieved by other means (e.g., using explicit loops), operators like `+=` and `*=`
-work by rebinding new values.
+that can be achieved by other means (e.g., using broadcasting or explicit loops), operators like `+=` and
+`*=` work by rebinding new values.
 
 ## [Asynchronous IO and concurrent synchronous writes](@id faq-async-io)