doc: Drop description of LogError messages as fatal

Remove documentation that says LogError should be used for "severe problems
that require the node (or a subsystem) to shut down entirely" because:

- This is not how `LogError` and `Level::Error` are used currently. Of 129
  current uses only 58 cases are fatal according to
  bitcoin#30348

- "[T]here's not much benefit in a log that says "hey this error is about to
  cause the node to stop" -- you already get that information by seeing
  "Shutdown: in progress..." immediately following." according to
  bitcoin#30347 (comment)

doc/developer-notes.md

@@ -26,6 +26,7 @@ Developer Notes
     - [Ignoring IDE/editor files](#ignoring-ideeditor-files)
 - [Development guidelines](#development-guidelines)
     - [General Bitcoin Core](#general-bitcoin-core)
+    - [Logging](#logging)
     - [Wallet](#wallet)
     - [General C++](#general-c)
     - [C++ data structures](#c-data-structures)
@@ -742,14 +743,13 @@ logging messages. They should be used as follows:
   attacker to fill up storage. Note that `LogPrintf(fmt, params...)` is
   a deprecated alias for `LogInfo`.
 
-- `LogError(fmt, params...)` should be used in place of `LogInfo` for
-  severe problems that require the node (or a subsystem) to shut down
-  entirely (e.g., insufficient storage space).
+- `LogError(fmt, params...)` should be used in place of `LogInfo` for severe
+  errors the node admin will need to address (e.g., failure to write data).
 
 - `LogWarning(fmt, params...)` should be used in place of `LogInfo` for
-  severe problems that the node admin should address, but are not
-  severe enough to warrant shutting down the node (e.g., system time
-  appears to be wrong, unknown soft fork appears to have activated).
+  unexpected conditions indicating potentially severe problems the node admin
+  should address (e.g. system time appears to be wrong, unknown soft fork
+  appears to have activated).
 
 - `LogTrace(BCLog::CATEGORY, fmt, params...)` should be used in place of
   `LogDebug` for log messages that would be unusable on a production