Improve manual in various ways (inputs, sort_by, foreach sections, etc.) (#2744)

- Add error/0 and mentions null input behavior (close #2231)
- Explain value iterator suffix syntax .foo[] (close #1047)
- Mention array slicing is also zero-based (close #2094)
- Add examples of input and inputs filters (close #2216, close #2470)
- Improve sort_by about multiple values (close #2103, close #2467, close #2474)
- Improve foreach section and simplify examples (close #1148, close #2169)
- Fix recurse/1 document on how it is identical using recurse/2 (close #2036, close #2412)
- Add non-string examples of index/1, rindex/1 (close #1422)
- Simplify the example of truncate_stream/1 (close #1736)

tests/jq.test

@@ -1232,6 +1232,14 @@ null
 [null,true,{"a":1}]
 []
 
+[1,error,2]
+null
+[1,2]
+
+[1,error(null),2]
+0
+[1,2]
+
 try error("\($__loc__)") catch .
 null
 "{\"file\":\"<top-level>\",\"line\":1}"