Update glabel(8) to clarify filesystem volume labels, device metadata identifiers, and generic GEOM labels.

[felix.the.red_gmail.com]

Summary

• Change description of manual page
• Split out filesystem, GPT and GEOM labels.
• Give an example for each filesystem of the program to use to set a label.
• Clarify examples.

To summarize the original PR:

• "Labelization" is not a real word. Please change the description of the manual page.
• glabel(8) is confusing and doesn't distinguish between filesystem labels and GEOM labels.
• The example doesn't appear to show glabel in action.

PR:262207

Test Plan

Check formatting
mandoc -Tascii lib/geom/label/glabel.8

Check for linter errors:
mandoc -Tlint lib/geom/label/glabel.8

[felix.the.red_gmail.com]

Remove empty Em.
Remove end-of-line space.