Improve documentation for creating and sending messages.

Add important information about Swift_RfcComplianceException and Swift_Validate.
merged branch Haprog/improve-docs (PR #326)

This PR was merged into the master branch.

Discussion
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Fix issue #304. Improve documentation for creating and sending messages.

Fix issue https://github.com/swiftmailer/swiftmailer/issues/304.
Add important information about ``Swift_RfcComplianceException`` and ``Swift_Validate``.

Commits
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e4787af Improve documentation for creating and sending messages.