docs(bloom-filter): correct k parameter terminology and add calculator link

Change description from "bits set" to "number of hash functions" to align
with standard Bloom filter terminology. Add link to Bloom filter calculator
to help users determine optimal bf and k values for their use case.

Author: tobyhede

docs/reference/index-config.md

@@ -71,7 +71,7 @@ The default match index options are:
 - `token_filters`: a list of filters to apply to normalize tokens before indexing.
 - `tokenizer`: determines how input text is split into tokens.
 - `bf`: The size of the backing [bloom filter](https://en.wikipedia.org/wiki/Bloom_filter) in bits. Defaults to `2048`.
-- `k`: The maximum number of bits set in the bloom filter per term. Defaults to `6`.
+- `k`: The number of hash functions to use per term (each sets one bit in the bloom filter). Defaults to `6`.
 
 **Token filters**
 
@@ -93,6 +93,8 @@ There are two `tokenizer`s provided: `standard` and `ngram`.
 This determines the maximum number of bits that will be set in the bloom filter per term.
 `k` must be an integer from `3` to `16` and defaults to `6`.
 
+To calculate optimal values for your use case, see this [Bloom filter calculator](https://di-mgt.com.au/bloom-calculator.html).
+
 **Caveats around n-gram tokenization**
 
 While using n-grams as a tokenization method allows greater flexibility when doing arbitrary substring matches, it is important to bear in mind the limitations of this approach.