Improve description of keys in tsvector

Improve description of keys in tsvector

If all the bits of a key in a tsvector are true (marked with ALLISTRUE),
gtsvectorout() would show the following description:
"0 true bits, 0 false bits"

This is confusing, as all the bits are true, but this would be
equivalent to the information if siglen is 0.

This commit improves the output so as "all true bits" show instead in
this case.  Alexander has proposed a regression test for pageinspect,
not included here as it is rather expensive compared to its coverage
value.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/17950-6c80a8d2b94ec695@postgresql.org