[ruby/timeout] Improve Timeout.timeout documentation formatting and typos

This commit makes a few minor improvements to the documentation of the `Timeout.timeout` method.
The changes include fixing typos and enhancing formatting for better readability.

We can see the latest version of the documentation here:
https://docs.ruby-lang.org/en/master/Timeout.html#method-c-timeout

Also, we can verify this change locally by running:

```shell
rdoc
open doc/Timeout.html
```

ruby/timeout@53ee559e15

Author: ybiquitous

lib/timeout.rb

@@ -212,17 +212,17 @@ def self.synchronize(mutex, &block)
   #         value of 0 or +nil+ will execute the block without any timeout.
   #         Any negative number will raise an ArgumentError.
   # +klass+:: Exception Class to raise if the block fails to terminate
-  #           in +sec+ seconds.  Omitting will use the default, Timeout::Error
+  #           in +sec+ seconds.  Omitting will use the default, Timeout::Error.
   # +message+:: Error message to raise with Exception Class.
-  #             Omitting will use the default, "execution expired"
+  #             Omitting will use the default, <tt>"execution expired"</tt>.
   #
   # Returns the result of the block *if* the block completed before
   # +sec+ seconds, otherwise raises an exception, based on the value of +klass+.
   #
   # The exception raised to terminate the given block is the given +klass+, or
   # Timeout::ExitException if +klass+ is not given. The reason for that behavior
-  # is that Timeout::Error inherits from RuntimeError and might be caught unexpectedly by `rescue`.
-  # Timeout::ExitException inherits from Exception so it will only be rescued by `rescue Exception`.
+  # is that Timeout::Error inherits from RuntimeError and might be caught unexpectedly by +rescue+.
+  # Timeout::ExitException inherits from Exception so it will only be rescued by <tt>rescue Exception</tt>.
   # Note that the Timeout::ExitException is translated to a Timeout::Error once it reaches the Timeout.timeout call,
   # so outside that call it will be a Timeout::Error.
   #
@@ -231,21 +231,21 @@ def self.synchronize(mutex, &block)
   # For those reasons, this method cannot be relied on to enforce timeouts for untrusted blocks.
   #
   # If a scheduler is defined, it will be used to handle the timeout by invoking
-  # Scheduler#timeout_after.
+  # Fiber::Scheduler#timeout_after.
   #
   # Note that this is both a method of module Timeout, so you can <tt>include
   # Timeout</tt> into your classes so they have a #timeout method, as well as
   # a module method, so you can call it directly as Timeout.timeout().
   #
   # ==== Ensuring the exception does not fire inside ensure blocks
   #
-  # When using Timeout.timeout it can be desirable to ensure the timeout exception does not fire inside an +ensure+ block.
-  # The simplest and best way to do so it to put the Timeout.timeout call inside the body of the begin/ensure/end:
+  # When using Timeout.timeout, it can be desirable to ensure the timeout exception does not fire inside an +ensure+ block.
+  # The simplest and best way to do so is to put the Timeout.timeout call inside the body of the +begin+/+ensure+/+end+:
   #
   #     begin
   #       Timeout.timeout(sec) { some_long_operation }
   #     ensure
-  #       cleanup # safe, cannot be interrupt by timeout
+  #       cleanup # safe, cannot be interrupted by timeout
   #     end
   #
   # If that is not feasible, e.g. if there are +ensure+ blocks inside +some_long_operation+,
@@ -263,17 +263,17 @@ def self.synchronize(mutex, &block)
   #       end
   #     }
   #
-  # An important thing to note is the need to pass an exception klass to Timeout.timeout,
-  # otherwise it does not work. Specifically, using +Thread.handle_interrupt(Timeout::ExitException => ...)+
+  # An important thing to note is the need to pass an exception +klass+ to Timeout.timeout,
+  # otherwise it does not work. Specifically, using <tt>Thread.handle_interrupt(Timeout::ExitException => ...)</tt>
   # is unsupported and causes subtle errors like raising the wrong exception outside the block, do not use that.
   #
   # Note that Thread.handle_interrupt is somewhat dangerous because if setup or cleanup hangs
   # then the current thread will hang too and the timeout will never fire.
   # Also note the block might run for longer than +sec+ seconds:
-  # e.g. some_long_operation executes for +sec+ seconds + whatever time cleanup takes.
+  # e.g. +some_long_operation+ executes for +sec+ seconds + whatever time cleanup takes.
   #
-  # If you want the timeout to only happen on blocking operations one can use :on_blocking
-  # instead of :immediate. However, that means if the block uses no blocking operations after +sec+ seconds,
+  # If you want the timeout to only happen on blocking operations, one can use +:on_blocking+
+  # instead of +:immediate+. However, that means if the block uses no blocking operations after +sec+ seconds,
   # the block will not be interrupted.
   def self.timeout(sec, klass = nil, message = nil, &block)   #:yield: +sec+
     return yield(sec) if sec == nil or sec.zero?