Add info to stringstore and vocab (#12471)

Author: thomashacker

website/docs/api/stringstore.mdx

@@ -8,6 +8,13 @@ Look up strings by 64-bit hashes. As of v2.0, spaCy uses hash values instead of
 integer IDs. This ensures that strings always map to the same ID, even from
 different `StringStores`.
 
+<Infobox variant ="warning">
+
+Note that a `StringStore` instance is not static. It increases in size as texts
+with new tokens are processed.
+
+</Infobox>
+
 ## StringStore.\_\_init\_\_ {id="init",tag="method"}
 
 Create the `StringStore`.

website/docs/api/vocab.mdx

@@ -10,6 +10,13 @@ The `Vocab` object provides a lookup table that allows you to access
 [`StringStore`](/api/stringstore). It also owns underlying C-data that is shared
 between `Doc` objects.
 
+<Infobox variant ="warning">
+
+Note that a `Vocab` instance is not static. It increases in size as texts with
+new tokens are processed.
+
+</Infobox>
+
 ## Vocab.\_\_init\_\_ {id="init",tag="method"}
 
 Create the vocabulary.