document Libc.FILE (#49908)

Adds some missing documentation for `Libc.FILE`, which is already
exported.

base/libc.jl

@@ -75,6 +75,34 @@ end
 
 ## FILE (not auto-finalized) ##
 
+"""
+    FILE(::Ptr)
+    FILE(::IO)
+
+A libc `FILE*`, representing an opened file.
+
+It can be passed as a `Ptr{FILE}` argument to [`ccall`](@ref) and also supports
+[`seek`](@ref), [`position`](@ref) and [`close`](@ref).
+
+A `FILE` can be constructed from an ordinary `IO` object, provided it is an open file. It
+must be closed afterward.
+
+# Examples
+```jldoctest
+julia> using Base.Libc
+
+julia> mktemp() do _, io
+           # write to the temporary file using `puts(char*, FILE*)` from libc
+           file = FILE(io)
+           ccall(:fputs, Cint, (Cstring, Ptr{FILE}), "hello world", file)
+           close(file)
+           # read the file again
+           seek(io, 0)
+           read(io, String)
+       end
+"hello world"
+```
+"""
 struct FILE
     ptr::Ptr{Cvoid}
 end

doc/src/base/libc.md

@@ -17,6 +17,7 @@ Base.Libc.time(::Base.Libc.TmStruct)
 Base.Libc.strftime
 Base.Libc.strptime
 Base.Libc.TmStruct
+Base.Libc.FILE
 Base.Libc.flush_cstdio
 Base.Libc.systemsleep
 ```