bug#3564: 23.0.94; compose-region doc string

[Kenichi Handa]

In article <FA8E5A4812B64B7AAAC196EE35260545@us.oracle.com>, "Drew Adams" 
<drew.adams@oracle.com> writes:

> 1. Typo: "stricking" -> "striking".
> 2. Please add an explanation of what "overstriking" and "stacking"
> mean.
 
How about this change?

--- composite.el.~1.47.~        2009-06-08 20:48:58.000000000 +0900
+++ composite.el        2009-06-15 15:39:21.000000000 +0900
@@ -185,8 +185,17 @@
 (defun compose-region (start end &optional components modification-func)
   "Compose characters in the current region.
 
-Characters are composed relatively, i.e. composed by overstricking or
-stacking depending on ascent, descent and other properties.
+Characters are composed relatively, i.e. composed by overstriking
+or stacking depending on ascent, descent and other metrics values
+of glyphs.
+
+For instance, if the region has three characters \"XYZ\", X is
+regarded as BASE glyph, and Y is displayed:
+  (1) above BASE if Y's descent is not positive,
+  (2) below BASE if Y's ascent is not positive,
+  (3) over BASE (at the same position of BASE) otherwise.
+and Z is displayed with the same rule while regarding the whole
+XY glyphs as BASE.
 
 When called from a program, expects these four arguments.
 
---
Kenichi Handa
handa@m17n.org