CODEC-310 - Documentation update for the org.apache.commons.codec.digest.* package

[yakovsh]

This PR:

• Documents that SecureRandom is used in Crypt, Md5Crypt and Sha2Crypt classes by changing Javadocs in these classes (by calling B64.randomSalt)
• Changes salt generation in UnixCrypt to use SecureRandom to match the other classes
• Update the userguide to list all functions from the digest package
• Changes the hyperlinks in the user guide from HTTP to HTTPS

See https://issues.apache.org/jira/browse/CODEC-310

[codecov-commenter]

Codecov Report

Merging #208 (22c3d4c) into master (fab2cf8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #208   +/-   ##
=========================================
  Coverage     92.21%   92.21%           
  Complexity     1740     1740           
=========================================
  Files            67       67           
  Lines          4584     4584           
  Branches        709      709           
=========================================
  Hits           4227     4227           
  Misses          244      244           
  Partials        113      113           

Files	Coverage Δ
...in/java/org/apache/commons/codec/digest/Crypt.java	100.00% <ø> (ø)
...java/org/apache/commons/codec/digest/Md5Crypt.java	100.00% <ø> (ø)
...ava/org/apache/commons/codec/digest/Sha2Crypt.java	94.02% <ø> (ø)
...ava/org/apache/commons/codec/digest/UnixCrypt.java	98.66% <100.00%> (ø)

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[garydgregory]

Patch applied. TY @yakovsh!