moved a subset of source file to the kern package

doc/Expr.adoc

@@ -1421,12 +1421,38 @@ Same as <<_add,add()>> but returns the product of the values of the parameters.
 [green]`24`
 
 ==== Module "os.file"
+The "os.file" module provides functions for working with files.
+
+Activation: +
+`{4sp}builtin "os.file"`
+
+Currently available functions:
+
+* <<_fileOpen,fileOpen()>>
+* <<_fileAppend,fileAppend()>>
+* <<_fileCreate,fileCreate()>>
+* <<_fileClose,fileClose()>>
+* <<_fileWriteText,fileWriteText()>>
+* <<_fileReadText,fileReadText()>>
+* <<_fileReadTextAll,fileReadTextAll()>>
+
+More functions will be added in the future.
+
+---
 
 ===== fileOpen()
+Syntax: +
+`{4sp}fileOpen(<file-path>) -> file-handle`
+
+Returns a file handle for the specified file path. The file is opened in read-write mode. If the file does not exist, it is created.
 
 ===== fileAppend()
 
 ===== fileCreate()
+Syntax: +
+`{4sp}fileCreate(<file-path>) -> file-handle`
+
+Creates or truncates the named _<file-path>_. If the file already exists, it is truncated. If the file does not exist, it is created with mode 0o666 (before umask). The associated file descriptor has mode [O_RDWR]. The directory containing the file must already exist.
 
 ===== fileClose()
 
@@ -1438,6 +1464,24 @@ Same as <<_add,add()>> but returns the product of the values of the parameters.
 
 
 ==== Module "string"
+This module provides functions for working with strings.
+
+Activation: +
+`{4sp}builtin "string"`
+
+
+Currently available functions:
+
+* <<_strJoin,strJoin()>>
+* <<_strSub,strSub()>>
+* <<_strSplit,strSplit()>>
+* <<_strTrim,strTrim()>>
+* <<_strStartsWith,strStartsWith()>>
+* <<_strEndsWith,strEndsWith()>>
+* <<_strUpper,strUpper()>>
+* <<_strLower,strLower()>>
+
+---
 
 ===== strJoin()
 Syntax: +