[ruby/prism] Document remaining encodings

https://github.com/ruby/prism/commit/b9510aed40

prism/encoding.c

@@ -4214,7 +4214,7 @@ pm_encoding_shift_jis_char_width(const uint8_t *b, ptrdiff_t n) {
 /**
  * This is the definition of all of the encodings that we support.
  */
-static const pm_encoding_t pm_encodings[PM_ENCODING_MAXIMUM] = {
+static const pm_encoding_t pm_encodings[] = {
     [PM_ENCODING_UTF_8] = {
         .name = "UTF-8",
         .char_width = pm_encoding_utf_8_char_width,

prism/encoding.h

@@ -213,8 +213,7 @@ typedef enum {
     PM_ENCODING_WINDOWS_1257,
     PM_ENCODING_WINDOWS_1258,
     PM_ENCODING_WINDOWS_31J,
-    PM_ENCODING_WINDOWS_874,
-    PM_ENCODING_MAXIMUM
+    PM_ENCODING_WINDOWS_874
 } pm_encoding_type_t;
 
 /**

test/prism/library_symbols_test.rb

@@ -84,7 +84,7 @@ module Prism
         assert_includes(symbols, "pm_version")
       end
       names(local_nm_symbols(@libprism_so)).tap do |symbols|
-        assert_includes(symbols, "pm_encoding_shift_jis_isupper_char")
+        assert_includes(symbols, "pm_encoding_utf_8_isupper_char")
       end
       # TODO: someone who uses this library needs to finish this test
     end