ocr: compat: make a non-existant ocrprogs config variable equivalent to "tesseract"

src/sampleconf/recoll.conf

@@ -905,7 +905,7 @@ snippetMaxPosWalk = 1000000
 #
 # <brief>Attempt OCR of PDF files with no text content.</brief>
 # <descr>This can be defined in subdirectories. The default is off because
-# OCR is so very slow. Will only do anything if ocrprogs is defined.</descr>
+# OCR is so very slow.</descr>
 # </var>
 #pdfocr = 0
 
@@ -946,10 +946,13 @@ snippetMaxPosWalk = 1000000
 # <brief>OCR modules to try.</brief>
 # <descr>The top OCR script will try to load the corresponding modules in
 # order and use the first which reports being capable of performing OCR on
-# the input file. Modules for tesseract and ABBYY FineReader are present in
-# the standard distribution.</descr>
+# the input file. Modules for tesseract (tesseract) and ABBYY FineReader
+# (abbyy) are present in the standard distribution. For compatibility with
+# the previous version, if this is not defined at all, the default value is
+# "tesseract". Use an explicit empty value if needed. A value of "abbyy
+# tesseract" will try everything.</descr>
 # </var>
-#ocrprogs = abbyy tesseract
+#ocrprogs = tesseract
 
 # <var name="ocrcachedir" type="dfn">
 # <brief>Location for caching OCR data.</brief>