[JIRA] Document issue linking methods

[dephekt]

Adds documentation for the following Jira methods:

• Jira.get_issue_link()
• Jira.create_issue_link()
• Jira.remote_issue_link()

These methods already exist, they are just currently undocumented. It also renames the titles for the currently-documented remote link methods to indicate they are only for handling remote links (e.g. links external to Jira).

Fixes #724.

[codecov-io]

Codecov Report

Merging #729 (c3e4def) into master (235af5c) will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #729      +/-   ##
==========================================
+ Coverage   36.59%   36.71%   +0.12%     
==========================================
  Files          31       32       +1     
  Lines        5545     5556      +11     
  Branches      794      794              
==========================================
+ Hits         2029     2040      +11     
  Misses       3433     3433              
  Partials       83       83              

Impacted Files	Coverage Δ
atlassian/__init__.py	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 235af5c...c3e4def. Read the comment docs.

[gonchik]

Thank you! @dephekt