tar: add module comment and references

lib/std/tar.zig

@@ -1,4 +1,21 @@
 const std = @import("std.zig");
+/// Tar archive is single ordinary file which can contain many files (or
+/// directories, symlinks, ...). It's build by series of blocks each size of 512
+/// bytes. First block of each entry is header which defines type, name, size
+/// permissions and other attributes. Header is followed by series of blocks of
+/// file content, if any that entry has content. Content is padded to the block
+/// size, so next header always starts at block boundary.
+///
+/// This simple format is extended by GNU and POSIX pax extensions to support
+/// file names longer than 256 bytes and additional attributes.
+///
+/// This is not comprehensive tar parser. Here we are only file types needed to
+/// support Zig package manager; normal file, directory, symbolic link. And
+/// subset of attributes: name, size, permissions.
+///
+/// GNU tar reference: https://www.gnu.org/software/tar/manual/html_node/Standard.html
+/// pax reference: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13
+
 const assert = std.debug.assert;
 
 pub const Options = struct {
@@ -193,7 +210,7 @@ pub const Header = struct {
     }
 };
 
-// Breaks string on first null char.
+// Breaks string on first null character.
 fn nullStr(str: []const u8) []const u8 {
     for (str, 0..) |c, i| {
         if (c == 0) return str[0..i];