DOC: fixes empty boxes + too wide:
- excel: code snippets too wide
- visualisation: for some reason now empty boxes when ipython code blocks with plt.figure were suppressed, but not when removing the semicolon
DOC: improve docstring of pd.melt
Merge pull request #6746 from jorisvandenbossche/doc-fixes

Doc fixes
PERF: perf improvements in timedelta conversions from integer dtypes
DOC: documented that .apply(func) executes func twice on the first time

Related issues are #2656, #2936 and #6753.
Merge pull request #6756 from anton-d/master

DOC: documented that .apply(func) executes func twice on the first time
Merge pull request #6754 from jreback/timedelta_perf

PERF: perf improvements in timedelta conversions from integer dtypes
DOC: correct alignment of GH6756
ENH: Panel.shift now uses NDFrame.shift.
PRF: Adding Panels.shift test

DOC: Panel.shift no longer drops periods.