Docs: Bang has other meaning for IO and RNG functions (JuliaLang#50138)

* Docs: Bang has other meaning for IO and RNG functions

I have seen multiple people ask why functions such as `write` or `rand` does not
end with a `!`, given that they mutate the input IO or RNG object.
Mention in the style guide that `!` means something slightly different for
functions taking IO or RNG arguments.

* Update doc/src/manual/style-guide.md

Co-authored-by: Thomas Christensen <tchr@mit.edu>

* Update doc/src/manual/style-guide.md

Co-authored-by: Thomas Christensen <tchr@mit.edu>

* Update doc/src/manual/style-guide.md

Co-authored-by: Lilith Orion Hafner <lilithhafner@gmail.com>

* Increment -> advance

---------

Co-authored-by: Thomas Christensen <tchr@mit.edu>
Co-authored-by: Lilith Orion Hafner <lilithhafner@gmail.com>

doc/src/manual/style-guide.md

@@ -119,6 +119,10 @@ with both copying and modifying forms (e.g., [`sort`](@ref) and [`sort!`](@ref))
 which are just modifying (e.g., [`push!`](@ref), [`pop!`](@ref), [`splice!`](@ref)).  It
 is typical for such functions to also return the modified array for convenience.
 
+Functions related to IO or making use of random number generators (RNG) are notable exceptions:
+Since these functions almost invariably must mutate the IO or RNG, functions ending with `!` are used to signify a mutation _other_ than mutating the IO or advancing the RNG state.
+For example, `rand(x)` mutates the RNG, whereas `rand!(x)` mutates both the RNG and `x`; similarly, `read(io)` mutates `io`, whereas `read!(io, x)` mutates both arguments.
+
 ## Avoid strange type `Union`s
 
 Types such as `Union{Function,AbstractString}` are often a sign that some design could be cleaner.