Update documentation in a couple of places.
Remove uses of <tt>...</tt> in a number of places in favor of more
appropriate markups.
Reindent all short comments to standard length.
Convert two more places where we use <code>...</code>.
Patch suggested by Xinzeng Feng <xf39@cornell.edu>: Fix a mistake in a code example.
Merge pull request #170 from bangerth/fix-formula

Patch suggested by Xinzeng Feng <xf39@cornell.edu>: Fix a mistake in a code example.
Merge pull request #160 from bangerth/doc-fixes-7

Update documentation in a couple of places.