Documention Corrections - Various date_time Pages

[Charles Brockman <cmbrockman@…>]

Here are some suggested changes for the date_time library documentation.

In \libs\date_time\xmldoc\gregorian.xml, the phrase
"If you are interested in learning about writing programs do specialized date calculations..."
should probably be
"If you are interested in learning about writing programs that do specialized date calculations..."

In \libs\date_time\xmldoc\date_class.xml:
"...it is not possible to construct and 'invalid' date."
should be
"...it is not possible to construct an 'invalid' date."

<entry>Get the day of the week (eg: Sunday, Monday, etc.</entry>
should be
<entry>Get the day of the week (Sunday, Monday etc.)</entry>
[closing the parentheses and removing the superfluous "eg"]

<entry>To <code>YYYY-mmm-DD</code> string where <code>mmm</code> 3 char month name.</entry>
should be
<entry>To <code>YYYY-mmm-DD</code> string where <code>mmm</code> is a 3 char month name.</entry>

In the row after
<entry valign="top" morerows="1"><screen>date operator-(date_duration) const</screen></entry>, I think the line should be changed from
<entry>Return a date by adding a day offset</entry>
to
<entry>Return a date by subtracting a day offset</entry>

Here are two items for \libs\date_time\xmldoc\snap_to_details.xml:
"The <link linkend="iterators_intro">month_iterator</link> is not effected by this issue..."
should be
"The <link linkend="iterators_intro">month_iterator</link> is not affected by this issue..."
[effected = accomplished, affected = influenced]

"This would cause the final result to be different that the starting date."
should be
"This would cause the final result to be different than the starting date."