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

Based on a request from @luke-jr and criteria from @mikehearn, this pull
replaces the the previous security text in the Avoiding Key Reuse
section with a slightly expanded version that also describes why
creating more than one signature with the same private key might be a
problem.

If this pull request is unsatisfactory and non-trivial changes are
required to make it satisfactory, I suggest that we simply delete the
existing security text and add describing ECDSA security in detail it to
our [todo list for the next version of the
guide.](https://github.com/saivann/bitcoin.org/wiki/Content-Suggestions)

Note: every occurrence of "ECDSA" in the guide is automatically linked to
the ECDSA Wikipedia entry which repeatedly explains the risks of lack of
entropy in *k* values---with the Sony PS3 key compromise and
SecureRandom implementation being used as real-life examples. Anyone who
is curious about the details after reading our short description need
only click the link to start learning more.