feat(docs): document identifier restrictions

CHANGELOG.md

@@ -71,6 +71,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed links in Chinese translation: PR [#1206](https://github.com/tact-lang/tact/pull/1206)
 - Added a note on 255 being the maximum number of messages that can be sent during action phase: PR [#1237](https://github.com/tact-lang/tact/pull/1237)
 - Added onchain metadata creation for NFTs and Jettons to the cookbook: PR [#1236](https://github.com/tact-lang/tact/pull/1236)
+- Document that identifiers cannot start with `__gen` or `__tact`, and cannot contain Unicode characters apart from the small subset `a-zA-Z0-9_`: PR [#1312](https://github.com/tact-lang/tact/pull/1312)
 
 ### Release contributors
 

docs/src/content/docs/book/expressions.mdx

@@ -167,12 +167,18 @@ Read more about working with `null{:tact}` on the dedicated page: [Optionals](/b
 
 ## Identifiers
 
-An identifier is a sequence of characters in the code that _identifies_ a [variable](/book/statements#let), [constant](/book/constants), [map](/book/maps) and a [function](/book/functions), as well as a [Struct][s], [Message][m], [contract](/book/contracts), [trait](/book/types#traits), or their fields and methods. Identifiers are case-sensitive and not quoted.
+An identifier is a sequence of characters in the code that _identifies_ a [variable](/book/statements#let), [constant](/book/constants), [map](/book/maps) and a [function](/book/functions), as well as a [Struct][s], [Message][m], [contract](/book/contracts), [trait](/book/types#traits), or their fields and methods. Identifiers are case-sensitive and not quoted. An identifier differs from a [string](#string-literals) in that a string is data, while an identifier is part of the code.
 
-In Tact, identifiers can contain latin lowercase letters (`a-z`), latin uppercase letters (`A-Z`), underscores (`_`) and digits ($\mathrm{0 - 9}$), but may not start with a digit. An identifier differs from a [string](#string-literals) in that a string is data, while an identifier is part of the code.
+In Tact, identifiers may contain Latin lowercase letters `a-z`, Latin uppercase letters `A-Z`, underscores `_`, and digits $\mathrm{0 - 9}$, but may not start with a digit. No other symbols are allowed, and Unicode identifiers are prohibited.
 
 Note, that when identifiers for [primitive types][p] start with an uppercase letter. Used-defined [composite types](/book/types#composite-types), such as [Structs][s] and [Messages][m] also must be capitalized.
 
+:::caution
+
+  All identifiers starting with `__gen` and `__tact` are not allowed and are instead reserved for internal use by the compiler.
+
+:::
+
 ## Instantiation
 
 You can create instances of the following types: