PARQUET-2241: Update wording of BYTE_STREAM_SPLIT encoding

[wgtmac]

Propose to explicitly state that no padding is allowed within a data page. This makes it easier for BYTE_STREAM_SPLIT decoder to decode page with nulls. In this way, it can simply get the number of encoded values by total_length_encoded_stream / K (4 for float and 8 for double). Otherwise, it has to decode def/rep levels to get exact number of non-null values.

[wgtmac]

cc @shangxinli @gszadovszky @ggershinsky @pitrou @emkornfield

[pitrou]

cc @wjones127

[mapleFU]

I think should we check that no more padding is added in all impl? At least, seems C++, Rust, parquet-mr didn't padding at the end of data.

[emkornfield]

Seems OK to me.