Make several minor documentation fixes

* Escaped SMTP in some places where it refers to the protocol, not the
  class.
* Separated `SMTP.start or SMTP#start`, so rdoc can link both methods.
* Fixed ruby example syntax, so rdoc will syntax highlight.

lib/net/smtp.rb

@@ -79,11 +79,11 @@ class SMTPUnsupportedCommand < ProtocolError
   # == What is This Library?
   #
   # This library provides functionality to send internet
-  # mail via SMTP, the Simple Mail Transfer Protocol. For details of
-  # SMTP itself, see [RFC5321] (http://www.ietf.org/rfc/rfc5321.txt).
-  # This library also implements SMTP authentication, which is often
+  # mail via \SMTP, the Simple Mail Transfer Protocol. For details of
+  # \SMTP itself, see [RFC5321] (http://www.ietf.org/rfc/rfc5321.txt).
+  # This library also implements \SMTP authentication, which is often
   # necessary for message composers to submit messages to their
-  # outgoing SMTP server, see
+  # outgoing \SMTP server, see
   # [RFC6409](http://www.ietf.org/rfc/rfc6503.txt),
   # and [SMTPUTF8](http://www.ietf.org/rfc/rfc6531.txt), which is
   # necessary to send messages to/from addresses containing characters
@@ -147,7 +147,7 @@ class SMTPUnsupportedCommand < ProtocolError
   #     smtp.send_message msgstr, 'from@address', 'to@address'
   #     smtp.finish
   #
-  # You can also use the block form of SMTP.start/SMTP#start.  This closes
+  # You can also use the block form of SMTP.start or SMTP#start.  This closes
   # the SMTP session automatically:
   #
   #     # using block form of SMTP.start
@@ -160,30 +160,31 @@ class SMTPUnsupportedCommand < ProtocolError
   # === HELO domain
   #
   # In almost all situations, you must provide a third argument
-  # to SMTP.start/SMTP#start. This is the domain name which you are on
+  # to SMTP.start or SMTP#start. This is the domain name which you are on
   # (the host to send mail from). It is called the "HELO domain".
   # The SMTP server will judge whether it should send or reject
   # the SMTP session by inspecting the HELO domain.
   #
-  #     Net::SMTP.start('your.smtp.server', 25
-  #                     helo: 'mail.from.domain') { |smtp| ... }
+  #     Net::SMTP.start('your.smtp.server', 25, helo: 'mail.from.domain') do |smtp|
+  #       smtp.send_message msgstr, 'from@address', 'to@address'
+  #     end
   #
   # === SMTP Authentication
   #
   # The Net::SMTP class supports three authentication schemes;
   # PLAIN, LOGIN and CRAM MD5.  (SMTP Authentication: [RFC2554])
   # To use SMTP authentication, pass extra arguments to
-  # SMTP.start/SMTP#start.
+  # SMTP.start or SMTP#start.
   #
   #     # PLAIN
-  #     Net::SMTP.start('your.smtp.server', 25
+  #     Net::SMTP.start('your.smtp.server', 25,
   #                     user: 'Your Account', secret: 'Your Password', authtype: :plain)
   #     # LOGIN
-  #     Net::SMTP.start('your.smtp.server', 25
+  #     Net::SMTP.start('your.smtp.server', 25,
   #                     user: 'Your Account', secret: 'Your Password', authtype: :login)
   #
   #     # CRAM MD5
-  #     Net::SMTP.start('your.smtp.server', 25
+  #     Net::SMTP.start('your.smtp.server', 25,
   #                     user: 'Your Account', secret: 'Your Password', authtype: :cram_md5)
   #
   class SMTP < Protocol