Added notes on closing a library. (JuliaLang#23489)

* Added notes on closing a library.

Fixes 23459

* shorten line lengths

* "can"/"could" consistency.

doc/src/manual/calling-c-and-fortran-code.md

@@ -873,6 +873,25 @@ mylibvar = Libdl.dlopen("mylib")
 ccall(@dlsym("myfunc", mylibvar), Void, ())
 ```
 
+## Closing a Library
+
+It is sometimes useful to close (unload) a library so that it can be reloaded.
+For instance, when developing C code for use with Julia, one may need to compile,
+call the C code from Julia, then close the library, make an edit, recompile,
+and load in the new changes. One can either restart Julia or use the
+`Libdl` functions to manage the library explicitly, such as:
+
+```julia
+lib = Libdl.dlopen("./my_lib.so") # Open the library explicitly.
+sym = Libdl.dlsym(lib, :my_fcn) # Get a symbol for the function to call.
+ccall(sym, ...) # Use the symbol instead of the (symbol, library) tuple (remaining arguments are the same).
+Libdl.dlclose(lib) # Close the library explicitly.
+```
+
+Note that when using `ccall` with the tuple input
+(e.g., `ccall((:my_fcn, "./my_lib.so"), ...)`), the library is opened implicitly
+and it may not be explicitly closed.
+
 ## Calling Convention
 
 The second argument to [`ccall`](@ref) can optionally be a calling convention specifier (immediately