Document the rest of the C API

This commit fills out documentation for all remaining functions in the C
API, and additionally enables "warn if undocumented" which will fail CI
since warnings are also treated as errors.

crates/c-api/doxygen.conf

@@ -778,7 +778,7 @@ WARNINGS               = YES
 # will automatically be disabled.
 # The default value is: YES.
 
-WARN_IF_UNDOCUMENTED   = NO
+WARN_IF_UNDOCUMENTED   = YES
 
 # If the WARN_IF_DOC_ERROR tag is set to YES, doxygen will generate warnings for
 # potential errors in the documentation, such as not documenting some parameters
@@ -901,7 +901,9 @@ EXCLUDE_PATTERNS       =
 EXCLUDE_SYMBOLS        = assertions \
     WASM_DECLARE_* \
     WASM_API_EXTERN \
+    WASI_API_EXTERN \
     WASMTIME_DECLARE_OWN \
+    WASI_DECLARE_OWN \
     WASMTIME_CONFIG_PROP \
     own