Clarify that double is only the default floating-point type in the docs (#5830)

Follow-up to review comments on #5825: number_float_t uses double
precision only by default, and the stated limits apply to double.
Also say "default type for X_t" instead of "default value for X_t".

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/string_t.md

@@ -32,7 +32,7 @@ JSON class into byte-sized characters during deserialization.
 
 ### Default type
 
-With the default values for `StringType` (`std::string`), the default value for `string_t` is `#!cpp std::string`.
+With the default values for `StringType` (`std::string`), the default type for `string_t` is `#!cpp std::string`.
 
 ### Encoding