Add description for new --dpi option in tesseract man page

Signed-off-by: Stefan Weil <sw@weilnetz.de>

doc/tesseract.1.asc

@@ -52,6 +52,12 @@ OPTIONS
 *-c* 'CONFIGVAR=VALUE'::
   Set value for parameter 'CONFIGVAR' to VALUE. Multiple *-c* arguments are allowed.
 
+*--dpi* 'N'::
+  Specify the resolution 'N' in DPI for the input image(s).
+  A typical value for 'N' is `300`. Without this option,
+  the resolution is read from the metadata included in the image.
+  If an image does not include that information, Tesseract tries to guess it.
+
 *-l* 'LANG'::
 *-l* 'SCRIPT'::
   The language or script to use.