add header doc comments to non-obvious util data structures

include/static_vector.h

@@ -3,6 +3,9 @@
 #include <cstdint>
 #include <cstring>
 
+// Fixed-capacity contiguous container: std::vector's interface with std::array's
+// storage. Used for response payloads sized at parse time but bounded at compile
+// time, where heap allocation is unavailable or undesirable.
 template <typename T, size_t Capacity>
 class static_vector {
     T m_data[Capacity];