PARQUET-2435: Clarify behavior of DELTA_BINARY_PACKED encoding (#231)

Address the issue of using more bits in the encoding than are used in
the underlying type being encoded.

Encodings.md

@@ -245,7 +245,9 @@ Subtractions in steps 1) and 2) may incur signed arithmetic overflow, and so
 will the corresponding additions when decoding. Overflow should be allowed
 and handled as wrapping around in 2's complement notation so that the original
 values are correctly restituted. This may require explicit care in some programming
-languages (for example by doing all arithmetic in the unsigned domain).
+languages (for example by doing all arithmetic in the unsigned domain). Writers
+must not use more bits when bit packing the miniblock data than would be required
+to PLAIN encode the physical type (e.g. INT32 data must not use more than 32 bits).
 
 The following examples use 8 as the block size to keep the examples short,
 but in real cases it would be invalid.