document finally in try/catch (#54269)

As noted [on
discourse](https://discourse.julialang.org/t/try-catch-online-help/113532),
the `try` docstring should at least briefly mention `finally`.

---------

Co-authored-by: Sukera <[email protected]>

base/docs/basedocs.jl

@@ -1058,13 +1058,28 @@ exception object to the given variable within the `catch` block.
 
 The power of the `try`/`catch` construct lies in the ability to unwind a deeply
 nested computation immediately to a much higher level in the stack of calling functions.
+
+A `try` or `try`/`catch` block can also have a [`finally`](@ref) clause that executes
+at the end, regardless of whether an exception occurred.  For example, this can be
+used to guarantee that an opened file is closed:
+```julia
+f = open("file")
+try
+    operate_on_file(f)
+catch
+    @warn "An error occurred!"
+finally
+    close(f)
+end
+```
+(`finally` can also be used without a `catch` block.)
 """
 kw"try", kw"catch"
 
 """
     finally
 
-Run some code when a given block of code exits, regardless
+Run some code when a given `try` block of code exits, regardless
 of how it exits. For example, here is how we can guarantee that an opened file is
 closed: