org-agenda.el: Minor typo fix and docstring enhancement

* org-agenda.el (org-agenda-start-on-weekday): Fix typo.
(org-agenda-start-day): Enhance docstring.

* org.texi (Weekly/daily agenda): Add `org-agenda-start-day'
and `org-agenda-start-on-weekday' to the variable index and
document them.

doc/org.texi

@@ -7664,11 +7664,16 @@ C-c a a}) you may set the number of days to be displayed.
 
 @vindex org-agenda-span
 @vindex org-agenda-ndays
+@vindex org-agenda-start-day
+@vindex org-agenda-start-on-weekday
 The default number of days displayed in the agenda is set by the variable
-@code{org-agenda-span} (or the obsolete @code{org-agenda-ndays}).  This
+@var{org-agenda-span} (or the obsolete @var{org-agenda-ndays}).  This
 variable can be set to any number of days you want to see by default in the
-agenda, or to a span name, such a @code{day}, @code{week}, @code{month} or
-@code{year}.
+agenda, or to a span name, such as @code{day}, @code{week}, @code{month} or
+@code{year}.  For weekly agendas, the default is to start on the previous
+monday (see @var{org-agenda-start-on-weekday}).  You can also set the start
+date using a date shift: @code{(setq org-agenda-start-day "+10d")} will
+start the agenda ten days from today in the future.
 
 Remote editing from the agenda buffer means, for example, that you can
 change the dates of deadlines and appointments from the agenda buffer.

lisp/org-agenda.el

@@ -1134,7 +1134,7 @@ Custom commands can set this variable in the options section."
 
 (defcustom org-agenda-start-on-weekday 1
   "Non-nil means start the overview always on the specified weekday.
-0 denotes Sunday, 1 denotes Monday etc.
+0 denotes Sunday, 1 denotes Monday, etc.
 When nil, always start on the current day.
 Custom commands can set this variable in the options section."
   :group 'org-agenda-daily/weekly
@@ -4097,7 +4097,9 @@ When EMPTY is non-nil, also include days without any entries."
 (defvar org-agenda-start-day nil  ; dynamically scoped parameter
   "Start day for the agenda view.
 Custom commands can set this variable in the options section.
-See the docstring of `org-read-date' on allowed values.")
+This is usually a string like \"2007-11-01\", \"+2d\" or any other
+input allowed when reading a date through the Org calendar.
+See the docstring of `org-read-date' for details.")
 (defvar org-starting-day nil) ; local variable in the agenda buffer
 (defvar org-arg-loc nil) ; local variable