Update warning message for file system operations

[dd32]

Update the message to indicate that WP_Filesystem should not be used for general file IO.

Description

The coding standards suggest using WP_Filesystem instead of all file-operations, which often sends developers down a path that they really shouldn't have.

This just clarifies that perhaps they shouldn't perform file operations, and to use WP_Filesystem for upgrade-routines (Which the classes are intended for).

This might get some pushback from some people who feel WP_Filesystem should be used if it exists, but it's an abstraction that applies for a constantly reducing set of sites, which run on read-only filesystems and require FTP to interact with the WordPress files. Generally, WP_Filesystem either uses direct IO, or, FTP which requires interactive prompting, which most plugins/themes that use WP_Filesystem are not handling anyway.

I will note it's possible for WP_Filesystem to be configured with hard-coded credentials, but that's even more rare.

Pushing people onto this class just doesn't benefit anyone.

I would almost suggest that the mention of WP_Filesystem should be removed entirely, and this just be left as a "You really shouldn't do file operations if this is running often" type warning.

Suggested changelog entry

Related issues/external references

Fixes #