Documentation for the fixed directive.

Based on conversation with johnw on IRC, I believe this text properly
documents the intended feature of the fixed directive.

doc/ledger3.texi

@@ -2047,6 +2047,37 @@ Closes block commands like @code{tag} or @code{comment}.
 @item fixed
 @c instance_t::fixed_directive in textual.cc
 
+A fixed block is used to set fixated prices (@pxref{Fixated prices}) for a series of
+transactions.  It's purely a typing saver, for use when entering many
+transactions with fixated prices.
+
+Thus, the following:
+@smallexample
+fixed CAD $0.90
+   2012-04-10 Lunch in Canada
+       Assets:Wallet            -15.50 CAD
+       Expenses:Food            15.50 CAD
+
+   2012-04-11 Second day Dinner in Canada
+       Assets:Wallet            -25.75 CAD
+       Expenses:Food            25.75 CAD
+endfixed
+@end smallexample
+is equivalent to this:
+@smallexample
+   2012-04-10 Lunch in Canada
+       Assets:Wallet            -15.50 CAD @{=$0.90@}
+       Expenses:Food            15.50 CAD  @{=$0.90@}
+
+   2012-04-11 Second day Dinner in Canada
+       Assets:Wallet            -25.75 CAD  @{=$0.90@}
+       Expenses:Food            25.75 CAD   @{=$0.90@}
+@end smallexample
+
+Note that ending a @samp{fixed} is done differently than other
+directives, as @samp{fixed} is closed with an @samp{endfixed} (i.e.,
+there is @strong{no space} between @samp{end} and @samp{fixed}).
+
 @item include
 @c instance_t::include_directive in textual.cc
 Include the stated file as if it were part of the current file.