docs: clarify comments

Author: DeRuina

README.md

@@ -66,6 +66,7 @@ Manual rotation (e.g. on `SIGHUP`):
 ```go
 import (
     "log"
+    "os"
     "os/signal"
     "syscall"
 
@@ -199,6 +200,7 @@ On each new log file creation, timberjack:
 | **Manual (custom reason)**   | `Logger.RotateWithReason(s)`     | When called | Immediate | No | N/A | **No** | `-<sanitized reason>` | Falls back to `Rotate()` behavior if `s` sanitizes to empty. |
 
 > **Time zone:** scheduling and filename timestamps use UTC by default, or local time if `LocalTime: true`.
+> **Sanitized reason:** lowercase; `[a-z0-9_-]` only,  trims edge, max 32. 
 
 ## ⚠️ Rotation Notes & Warnings
 

timberjack.go

@@ -81,7 +81,7 @@ type rotateAt [2]int
 // Backups use the log file name given to Logger, in the form:
 // `name-timestamp-<reason>.ext` where `name` is the filename without the extension,
 // `timestamp` is the time of rotation formatted as `2006-01-02T15-04-05.000`,
-// `reason` is "size" or "time" (or "manual" for explicit Rotate calls), and `ext` is the original extension.
+// `reason` is "size" or "time" (Rotate/auto), or a custom tag (RotateWithReason), and `ext` is the original extension.
 // For example, if your Logger.Filename is `/var/log/foo/server.log`, a backup created at 6:30pm on Nov 11 2016
 // due to size would use the filename `/var/log/foo/server-2016-11-04T18-30-00.000-size.log`.
 //