Merge pull request #6739 from jorisvandenbossche/doc-fixes

DOC: small doc fixes (build warning in sym_diff + formatting in release notes)
BUG: MPL Plot cannot make loglog option worked
DOC: minor documenation changes
Merge pull request #6722 from sinhrks/loglog

BUG: MPLPlot cannot make loglog keyword worked
API: all offset operations now return Timestamp types (rather than datetime), Business/Week frequencies were incorrect (GH4069)
Merge pull request #6743 from jreback/freq

API: all offset operations now return Timestamp types (rather than datetime) (GH4069)
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
TST: windows dtype correction
BUG: fix NDFrame.as_blocks() for sparse containers

SparseBlocks don't consolidate, so previous implementation silently
dropped all but the last blocks for given dtype.

Also, drop the 'columns' kwarg, there's reindex for that.
Merge pull request #6747 from dalejung/quick_shift_fixes

Quick shift fixes
Merge pull request #6748 from immerrr/fix-as-blocks-for-sparse

BUG: fix NDFrame.as_blocks() for sparse containers
BUG: NaTType can be unpickled properly
Fix SQLLegacy to allow parameters as mapping
Add SQL read_parameters tests
Merge pull request #6709 from gpoulin2/master

BUG: Allow mapping as parameters for SQL DBAPI2
Merge pull request #6746 from jorisvandenbossche/doc-fixes

Doc fixes