This is a problem with lots of feature-rich software, even with meticulously-documented APIs. What we need is reverse-indexed documentation. That is, an extensive API reference is only useful for someone who already knows what functions are in the API and just needs to remember how to use them. But even the most thorough API reference does nothing to promote discovering new functionality. This is often left to the authors, who then have to go about writing a User's Guide that gradually explains concepts, idioms, etc. in prose.

Thorough User's Guides are rare because they are tough to write, and even tougher to write well. Users don't often have the time to read through potentially hundreds of pages of prose to find what they're looking for. We need a better way to let users search or browse for concepts, and then be given a list of the functions that implement each concept.

That is, addition to documentation like:

    size_t strlen(const char * s);
      RETURN: Length of string s.

    size_t strnlen(const char * s, size_t maxlen);
      RETURN: Length of string s, or maxlen (whichever is smaller).
      NOTE: Stops reading after maxlen.

    char * stpcpy(char * dst, const char * src);
      Copy src to dst.
      RETURN: pointer to trailing '\0' of dst, or dst[n] if no trailing NUL.
      NOTE: Undefined behavior if dst and src overlap.

    char * stpncpy(char * dst, const char * src, size_t len);
      Copy up to len bytes from src to dst.
      RETURN: pointer to trailing '\0' of dst, or dst[n] if no trailing NUL.
      NOTE: Undefined behavior if dst and src overlap.

    char * strcpy(char * dst, const char * src);
      Copy src to dst.
      RETURN: dst.
      NOTE: Undefined behavior if dst and src overlap.

    char * strncpy(char * dst, const char * src, size_t len);
      Copy up to len bytes from src to dst
      RETURN: dst.
      NOTE: Undefined behavior if dst and src overlap.

    strcpy
      TAGS: "concept":"data type":"text", "concept":"attribute":"length",
            ".input":"array", ".input":"char", ".input":"pointer",
            ".return":"string", ".return":"pointer"
    strncpy
      TAGS: "concept":"data type":"text", "concept":"attribute":"length",
            "concept":"data type":"array":"max-length operator",
            ".input":"array", ".input":"char", ".input":"pointer",
            ".return":"array", ".return":"pointer"

    concept
    └─ data type
       └─ text
       └─ array (conceptual)
          └─ max-length operator
    └─ attribute
       └─ length / size
    data structure
    └─ char
    └─ array (implementation)
    └─ pointer
    

    '"concept":"data type":"text" AND "concept":"attribute":"length"'

It seems like a PITA at first, but I'm pretty sure tagging functions in their docstrings is easier than writing a whole new User's Guide.