ed: Improve F with repeated searches

Make F search from curpos+1 so that it's possible to search the
same word we've just found a second time. Previously, it would find
the word under the cursor.

Also, improve docs a bit.

blk/101

@@ -1,16 +1,15 @@
-T ( n -- ): select line n for editing.
-P xxx: put typed line on selected line.
-U xxx: insert typed line on selected line.
-F xxx: find typed string in block.
-I xxx: insert typed string at cursor.
-E: Delete previously found string.
-
-Note that "I" shadows core word. Use "i" to access core word.
-
-
-
-
-
-
-
+There are two buffers, IBUF (insert buffer) and FBUF (find
+buffer). They are used as a typing target for the actions
+described below. They both have a peculiar logic: when typing
+is expected to fill a buffer, an empty value means "reuse
+previous value". For example, typing "I foo I " inserts
+"foofoo".
 
+T ( n -- ): select line n for editing.
+P xxx: put typed IBUF on selected line.
+U xxx: insert typed IBUF on selected line.
+F xxx: find typed FBUF in block, starting from current
+       position+1. If not found, don't move.
+I xxx: insert typed IBUF at cursor. "I" shadows core word. Use
+       "i" to access it.
+E: From cursor, delete X characters, X = length of FBUF.

blk/107

@@ -1,5 +1,5 @@
 : _F ( F without _type and _pln. used in VE )
-    FBUF EDPOS @ _cpos ( a1 a2 )
+    FBUF EDPOS @ _cpos 1+ ( a1 a2 )
     BEGIN
         C@+ ROT ( a2+1 c2 a1 ) C@+ ROT ( a2+1 a1+1 c1 c2 )
         = NOT IF DROP FBUF THEN ( a2 a1 )