[PATCH][DOC][MINOR] Fix incorrect lexeme limit in textsearch docs

Hello,

A minor doc patch for this page
https://www.postgresql.org/docs/current/textsearch-limitations.html
and this line

*- The number of lexemes must be less than 2^64*

Docs wrongly claim "lexemes must be < 2^64" but the actual constraint is
1 MB total storage (MAXSTRPOS), and no 2^64 check exists in the code.

From src/include/tsearch/ts_type.h:

> #define MAXSTRPOS ( (1<<20) - 1) // 1,048,575 bytes
>
> typedef struct {
> int32 size; // number of lexemes
> ...
> } TSVectorData;

The attached patch:
- Removes the incorrect 2^64 claim
- Clarifies this means "distinct lexemes in a single tsvector value"

Thanks,
Dharin