Demonstrate rethrow() as best practice in try/catch docs (#59729)

Currently, exception type filtering is missing from the `try` keyword
docstrings, and proper demonstration of `rethrow` is missing from the
Control Flow manual page. This PR addresses these oversights.

Author: BioTurboNick

base/docs/basedocs.jl

@@ -1063,6 +1063,20 @@ end
 The syntax `catch e` (where `e` is any variable) assigns the thrown
 exception object to the given variable within the `catch` block.
 
+```julia
+try
+    a_dangerous_operation()
+catch e
+    if isa(e, EOFError)
+        @warn "The operation failed - EOF."
+    elseif isa(e, OutOfMemoryError)
+        @warn "The operation failed - OOM."
+    else
+        rethrow() # ensure other exceptions can bubble up the call stack
+    end
+end
+```
+
 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.
 

doc/src/manual/control-flow.md

@@ -785,6 +785,8 @@ julia> sqrt_second(x) = try
                sqrt(complex(x[2], 0))
            elseif isa(y, BoundsError)
                sqrt(x)
+           else
+               rethrow() # ensure other exceptions can bubble up the call stack
            end
        end
 sqrt_second (generic function with 1 method)
@@ -803,8 +805,18 @@ ERROR: DomainError with -9.0:
 sqrt was called with a negative real argument but will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).
 Stacktrace:
 [...]
+
+julia> sqrt_second([1 nothing])
+ERROR: MethodError: no method matching sqrt(::Nothing)
+The function `sqrt` exists, but no method is defined for this combination of argument types.
+[...]
 ```
 
+Use [`rethrow`](@ref) as above to continue unwinding the stack with the original exception so that
+higher-level exception handlers can deal with the exception. When filtering by exception type
+as above, it is often important to include `else rethrow()` so that other types of exceptions
+are not hidden from the caller.
+
 Note that the symbol following `catch` will always be interpreted as a name for the exception,
 so care is needed when writing `try/catch` expressions on a single line. The following code will
 *not* work to return the value of `x` in case of an error:
@@ -827,7 +839,7 @@ end
 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. There are situations where
 no error has occurred, but the ability to unwind the stack and pass a value to a higher level
-is desirable. Julia provides the [`rethrow`](@ref), [`backtrace`](@ref), [`catch_backtrace`](@ref)
+is desirable. Julia provides the [`backtrace`](@ref), [`catch_backtrace`](@ref)
 and [`current_exceptions`](@ref) functions for more advanced error handling.
 
 ### `else` Clauses