docs: update description of letter groups

Make it linkable by moving them under separate heading.
Update behavior description of applied rules for rule groups
and letter groups.
Add note about SetLetterBitsUTF8() for redefining standard letter
groups.

docs/dictionary.md

@@ -4,6 +4,7 @@
 - [Phoneme names](#phoneme-names)
 - [Pronunciation Rules](#pronunciation-rules)
   - [Rule Groups](#rule-groups)
+  - [Letter Groups](#letter-groups)
   - [Rules](#rules)
   - [Special Characters in \<phoneme string\>](#special-characters-in-phoneme-string)
   - [Special Characters in Both \<pre\> and \<post\> ](#special-characters-in-both-pre-and-post)
@@ -97,12 +98,25 @@ The rules are organized in groups, each starting with a `.group` line:
   They would not be needed for a language which has regular spelling rules. The
   first character can only be an ascii character (less than 0x80).
 
+**notes about rule groups**
+
+When matching a word, firstly the 2-letter group for the two letters at
+the current position in the word (if such a group exists) is searched,
+and then the single-letter group. The highest scoring rule in either of
+those two groups is used.
+
 * `.group`
   A group for other characters which don't have their own group.
 
 * `.replace`
   See section [Character Substitution](#character-substitution).
 
+### Letter groups
+
+Specific group of rules is declaration of letter sequences with some common
+feature of letters for particular language. It may be used as a placeholder
+of prefixes/infixes of words (in prerules) or infixed/postfixes in (postrules).
+
 * `.L<nn>`
   Defines a group of letter sequences, any of which can match with `Lnn` in a
   pre or post rule (see below). nn is a 2 digit decimal number in the range 01
@@ -113,12 +127,11 @@ The rules are organized in groups, each starting with a `.group` line:
 
 There can be up to 200 items in one letter group.
 
-When matching a word, firstly the 2-letter group for the two letters at
-the current position in the word (if such a group exists) is searched,
-and then the single-letter group. The highest scoring rule in either of
-those two groups is used.
+When matching a word, firstly the group containing most letters is checked at
+the current position in the word (if such a group exists), then shorter ones
+till to the single-letter groups. The highest scoring rule of matching group is used.
 
-`~` Letter in letter group means, that there can be no letter in this group 
+`~` Letter in letter group means, that there can be no letter in this group
     in the pre- or post- rule.
 
 _Example with prerule group:_
@@ -531,5 +544,5 @@ usually have specific meaning for each particular language.
 file by calling `SetLetterBits()` function from (usually) `NewTranslator()` function.
 Note, that letters should be stored as array of chars, thus multibyte
 unicode letters should be transposed using `transpose_min` and `transpose_max` parameters
-of particular `Translator` structure.
+of particular `Translator` structure, or using `SetLetterBitsUTF8()` function.