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

[harding]

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.

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.

[saivann]

As far as I'm concerned, this looks good to me.

[harding]

@luke-jr commit 1885c15 addresses both of your issues. Thanks!

[harding]

Merging this into pull #393. @luke-jr: further comments or (even better) pull requests are welcome. Thanks!

[saivann]

@harding Thanks!