bip159: Clarify that there is only one threshold

[maflcko]

@jonasschnelli

Changes:

• The last section mentions "thresholds", while only a single threshold of 244 is specified in the bip. Clarify by changing to "threshold"
• Remove internal Bitcoin Core implementation detail. This is not relevant for a bip.
• Remove word "additional" when referring to safety buffer, since the safety buffer is the only buffer and not an additional one.
• Remove empty section "references"
• Add missing link to signaling implementation.

[jonasschnelli]

Thanks!
ACK f455a24da7b098d390b6405ae19826d0c5684c3e

[maflcko]

Added a link to the signaling implementation after your review.

[jonasschnelli]

Also ACK fa9a4f3