Doc: clearify that we don't support truncate multi-byte chars [ci skip]

Author: CarterLi

src/data/help_format.txt

@@ -11,6 +11,7 @@ See "fastfetch -h title-format" for all supported tags.
 In 2.17.0 or newer, a truncation length can be specified using syntax '<arg>:<trunc-length>`
 For example: "--title-format '{user-name:5}'" will truncate user name into 5-length string.
 If '<trunc-length>' is negative, an ellipsis … will be appended at the end when the original string is truncated.
+Note: The string length is counted in raw bytes. Multi-byte unicode charactors and ANSI escape codes are not taken into account.
 
 If the value index is missing, meaning the placeholder is "{}", an internal counter sets the value index.
 This means that the format string "Values: {1} ({2})" is equivalent to "Values: {} ({})".