Fix description of (move-to-column <n> t) when column <n> is inside a tab

This fixes bug #35647.  State that when indent-tabs-mode is non-nil, spaces
are inserted before the tab rather than the tab being replaced by spaces.

* doc/lispref/text.texi (columns)
* src/indent.c (move-to-column): Make the above documentation amendment.

doc/lispref/text.texi

@@ -2245,9 +2245,11 @@ If it is impossible to move to column @var{column} because that is in
 the middle of a multicolumn character such as a tab, point moves to the
 end of that character.  However, if @var{force} is non-@code{nil}, and
 @var{column} is in the middle of a tab, then @code{move-to-column}
-converts the tab into spaces so that it can move precisely to column
-@var{column}.  Other multicolumn characters can cause anomalies despite
-@var{force}, since there is no way to split them.
+either converts the tab into spaces (when @code{indent-tabs-mode} is
+@code{nil}), or inserts enough spaces before it (otherwise), so that
+point can move precisely to column @var{column}.  Other multicolumn
+characters can cause anomalies despite @var{force}, since there is no
+way to split them.
 
 The argument @var{force} also has an effect if the line isn't long
 enough to reach column @var{column}; if it is @code{t}, that means to

src/indent.c

@@ -983,9 +983,10 @@ If specified column is within a character, point goes after that character.
 If it's past end of line, point goes to end of line.
 
 Optional second argument FORCE non-nil means if COLUMN is in the
-middle of a tab character, change it to spaces.
-In addition, if FORCE is t, and the line is too short to reach
-COLUMN, add spaces/tabs to get there.
+middle of a tab character, either change it to spaces (when
+`indent-tabs-mode' is nil), or insert enough spaces before it to reach
+COLUMN (otherwise).  In addition, if FORCE is t, and the line is too short
+to reach COLUMN, add spaces/tabs to get there.
 
 The return value is the current column.  */)
   (Lisp_Object column, Lisp_Object force)