ld65.sgml: document '--allow-multiple-definition' switch

doc/ld65.sgml

@@ -71,6 +71,7 @@ Short options:
   -vm                   Verbose map file
 
 Long options:
+  --allow-multiple-definition   Allow multiple definitions
   --cfg-path path               Specify a config file search path
   --config name                 Use linker config file
   --dbgfile name                Generate debug information
@@ -98,6 +99,14 @@ Here is a description of all of the command-line options:
 
 <descrip>
 
+  <tag><tt>--allow-multiple-definition</tt></tag>
+
+  Normally when a global symbol is defined multiple times, ld65 will
+  issue an error and not create the output file. This option lets it
+  silently ignore this fact and continue. The first definition of a
+  symbol will be used.
+
+
   <label id="option--start-group">
   <tag><tt>-(, --start-group</tt></tag>