When Emacs loads functions and variables from a byte-compiled file, it normally does not load their documentation strings into memory. Each documentation string is dynamically loaded from the byte-compiled file only when needed. This saves memory, and speeds up loading by skipping the processing of the documentation strings.
This feature has a drawback: if you delete, move, or alter the compiled file (such as by compiling a new version), Emacs may no longer be able to access the documentation string of previously-loaded functions or variables. Such a problem normally only occurs if you build Emacs yourself, and happen to edit and/or recompile the Lisp source files. To solve it, just reload each file after recompilation.
Dynamic loading of documentation strings from byte-compiled files is
determined, at compile time, for each byte-compiled file. It can be
disabled via the option byte-compile-dynamic-docstrings.
If this is non-nil, the byte compiler generates compiled files
that are set up for dynamic loading of documentation strings.
To disable the dynamic loading feature for a specific file, set this
option to nil in its header line (see Local Variables in Files in The GNU Emacs Manual), like this:
-*-byte-compile-dynamic-docstrings: nil;-*-
This is useful mainly if you expect to change the file, and you want Emacs sessions that have already loaded it to keep working when the file changes.
Internally, the dynamic loading of documentation strings is accomplished by writing compiled files with a special Lisp reader construct, ‘#@count’. This construct skips the next count characters. It also uses the ‘#$’ construct, which stands for the name of this file, as a string. Do not use these constructs in Lisp source files; they are not designed to be clear to humans reading the file.