Add comments on coding-category-utf-8,

Add comments on coding-category-utf-8,
coding-category-utf-16-be, and coding-category-utf-16-le.
(coding_category_name): Include "coding-category-utf-8",
"coding-category-utf-16-be", and "coding-category-utf-16-le".
(UTF_8_1_OCTET_P) (UTF_8_EXTRA_OCTET_P) (UTF_8_2_OCTET_LEADING_P)
(UTF_8_3_OCTET_LEADING_P) (UTF_8_4_OCTET_LEADING_P)
(UTF_8_5_OCTET_LEADING_P) (UTF_8_6_OCTET_LEADING_P): New macros.
(detect_coding_utf_8): New function.
(UTF_16_INVALID_P) (TF_16_HIGH_SURROGATE_P)
(UTF_16_LOW_SURROGATE_P): New macros.
(detect_coding_utf_16): New function.
(detect_coding_mask): Fix bug of returning wrong mask bits in the
case that detect_coding_XXX returns a mask not set in
priorities[i].
(detect_eol_type_in_2_octet_form): New function.
(detect_eol): If cooding->category_idx is for UTF-16, call
detect_eol_type_in_2_octet_form instead of dectect_eol_type.
(detect_coding_system): Don't include `nil' coding-system in the
result.
(Fupdate_coding_systems_internal): Update all coding-categories.