Add Comparison-Based Attacks To Reasons Not To Reuse Keys

_includes/guide_transactions.md:

* Expand the security part of the Avoiding Key Reuse subsection to also
  describe why creating more than one signature with the same private
  key might be a problem. Based on feedback from @luke-jr. Thanks!
Mention Side-Channel Attacks In Addition To Lack-Of-Entropy Attacks
Merge pull request #395 from harding/docsupdate-sigsecurity

Devel Docs: Add Comparison-Based Attacks To Reasons Not To Reuse Keys