Update tesseract man page about both OCR engines in tesseract 4

doc/tesseract.1.asc

@@ -17,6 +17,12 @@ between 1985 and 1995. In 1995, this engine was among the top 3 evaluated by
 UNLV. It was open-sourced by HP and UNLV in 2005, and has been developed
 at Google since then.
 
+Tesseract 4 adds a new neural net (LSTM) based OCR engine which is focused
+on line recognition, but also still supports the legacy Tesseract OCR engine of 
+Tesseract 3 which works by recognizing character patterns. Compatibility with 
+Tesseract 3 is enabled by --oem 0. It also needs traineddata files which support 
+the legacy engine, for example those from the tessdata repository.
+
 
 IN/OUT ARGUMENTS
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