Hey Calcite Devs, I am currently working on CALCITE-6215 <https://issues.apache.org/jira/browse/CALCITE-6215>. During my work, I have noticed that certain functions have multiple variations with different parameter types in their respective databases. For example, in PostgreSQL, the to_char function supports multiple forms such as to_char(timestamp, text), to_char(interval, text), and to_char(numeric_type, text).
However, the description in Calcite is not clear enough. For instance, the reference.md document describes the to_char function as follows: | m o p | TO_CHAR(timestamp, format) | Converts *timestamp* to a string using the format *format*. This description may not provide enough clarity for users to understand the usage of each function across different databases. I suggest considering adding specific links to the corresponding database functions in the reference.md document to enhance its completeness. This would allow users to easily access the documentation for the respective database functions. Thanks as always, Cancai Cai https://www.postgresql.org/docs/16/functions-formatting.html#FUNCTIONS-FORMATTING-DATETIME-TABLE
