Docs for reading external command from custom buffer (#53652)

---------

Co-authored-by: Sukera <11753998+Seelengrab@users.noreply.github.com>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Kristoffer Carlsson <kcarlsson89@gmail.com>

doc/src/manual/running-external-programs.md

@@ -79,6 +79,18 @@ julia> `echo "foo bar"`[2]
 "foo bar"
 ```
 
+You can also pass a `IOBuffer`, and later read from it:
+
+```jldoctest
+julia> io = PipeBuffer(); # PipeBuffer is a type of IOBuffer
+
+julia> run(`echo world`, devnull, io, stderr);
+
+julia> readlines(io)
+1-element Vector{String}:
+ "world"
+```
+
 ## [Interpolation](@id command-interpolation)
 
 Suppose you want to do something a bit more complicated and use the name of a file in the variable